i'm the responsible one. craig melvin: a working mom, new at the office. she loved her job and really loved her handsome, young boss. the thing that i never wanted to face was the hurt that i was going to cause. craig melvin: a passionate 9:00 to 5:00 affair. the problem-- she was married. and so was he. i grabbed his face, and i was like, "you know what, i love you. i'm not going anywhere. just tell me what's happening." craig melvin: cheating husbands, scheming wives-- suburbs are full of secrets. but being unfaithful to your spouse isn't a crime, is it? that would come next. it was just like every emotion possible all in one second. i went, [gasps], oh my god. craig melvin: murder in the dark-- who was behind it, and who would pay? when you see him on the video, he's armed and ready. craig melvin: two couples, two families, and the single moment that shattered it all. i knew, one day, that this was all going to come out. hello, and welcome to "dateline." it's a cautionary tale that started with two seemingly happy couples in suburban idaho. then their lives collided in a twisted love triangle that ended with gunfire in a parking lot. the showdown left one husband dead and the other injured. for investigators, figuring out who fired the fatal shots meant untangling the relationships at the heart of the crime, which were complicated. here's keith morrison with "deadly desire." keith morrison: look at this place now-- so ordinary, with its pharmacy, its grocery store, its carefully tended parking places so alike to suburban strip malls from bismarck to bakersfield. but that night? that cold night, a heat gathered here-- sweet, terrifying, doomed. for the longest time, i couldn't go anywhere because i'm thinking everybody's looking at me. keith morrison: this is the story of two married couples, of the advice they can offer others now that it's too late for them. i think, if i could tell them anything, it would be put your family first. keith morrison: but of course, that's not what happened. and this parking lot, there'll be a body here before we're done. keith morrison: careful when you stir the hot pot of desire. the place is meridian idaho, little brother to bigger boise here in the foothills of the majestic rockies. this is where they got to their perfect place, the end of their rainbow. it was 2006, and luck was on their side. they had just moved from southern california, and they were happy. they were fulfilled at work. they had two beautiful children. they had everything they ever wanted. they were rob and kandi hall, one of those charmed couples who'd fallen in love at first sight. in their case, first sight meant admiring each other in the gym. we were out, i think, that weekend, and then we never stopped. what was it about that relationship? what was it about him that you liked so much, that was so good? we just connected instantly. keith morrison: by the time rob and kandi moved to idaho, they'd been together for years, had two teenage daughters. rob landed an excellent job as a computer specialist at the ada county sheriff's office, specializing in vehicle locators inside squad cars. he loved every minute of his job. keith morrison: and as if it was a sign that this is where they belonged, kandi's career as a paralegal took off too. she was a natural, could and did sell her confidence and skills to anybody who walked in the door. it didn't hurt that she was pretty. keith morrison: no, it didn't. and her co-worker, sofia serna, idolized kandi. you became close. oh, we became very close. she was like a mother to me. keith morrison: yes, it was all just about perfect. and then who knows why these things happen exactly. they just do. and no one imagined why, would they, how this thing was going to end. about eight months into his new job, rob started traveling for work. nothing unusual about that, of course. but soon he seemed to be staying away from home a little longer than he really had to, started snapping at her, too, kandi said, about little things, not like the old rob at all. so at this point, you begin to suspect something. yeah. i start thinking, what is going on? why are you acting like this? keith morrison: and then one night, after a late flight in from california, rob lay down beside his wife in bed and it all came tumbling out. he just started to cry. and he said, "i'm having an affair." and i laid my head on his chest, and i said, "rob, please, just fix it." so you didn't get upset? you didn't say-- you didn't yell? you didn't scream? you didn't cry? you didn't do anything? that's what shocked him. you know, the typical response is, get the hell out. you throw the clothes out the window. oh, no. the last person on this planet that you would think to have an affair would be robert. keith morrison: but of course, it was devastating, crippling. every day she went to work, and every day sofia saw her friend turn herself inside out and just seem to wither. i watched her go through misery. did she still love him? yes, dearly. and wanted the marriage to continue in spite of the affair? yes, she did. she didn't want to believe in her head that he found something in this woman that wasn't in kandi. keith morrison: well, he felt bad about it, agreed to go through counseling with kandi, but-- after he confessed to you and you said, fix it, he didn't. he didn't know what he wanted. come on, no. i mean, he wanted to keep going with the affair. that's what he wanted. i for sure told him to stop. stop right now or i'll stamp my foot and hold my breath. yeah. and he kept doing it. yeah. it was my fault. he had a void because of me. that's what i was thinking. what do you think the void might have been? or what did you think the void? i just was boring. keith morrison: boring old kandi hall, rejected, apparently unlovable, and nearly 40. and then one day at work, kandi was introduced to a recent law school graduate who was looking to staff his new office, a boyishly handsome, smart as a whip, cocky young lawyer. his name was emmett corrigan. my friend, she said, emmett, you've got to meet kandi. i mean, she's just as passionate and aggressive as you are, and she would be great for you. keith morrison: and something suddenly lifted in kandi hall. by the time those words had left her friend's mouth, kandi knew. she just knew. kandi hall has a decision to make, and it will have consequences she never intended, deadly ones. coming up, three unfaithful spouses and their lies candy hall will have a decision to make. and it will have consequences. three unfaithful spouses and the lies are piling up. >> i. >> when dateline continues. co of amazing fragrance per dual pack. now that's a breath of fresh air wick. looking for a smarter way to mop? 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[ gasps ] [ chuckling ] good job, junior. way to go. [ chuckling ] [ speaking minionese ] keith morrison: in boise, idaho, inside this law office in the fall of 2010 was a paralegal whose charmed life was falling apart. kandi hall was an unhappy woman. her marriage was dying or dead. her 40th birthday was bearing down like a chinese bullet train. and then, one day, it got worse. kandi's boss told her she was also unemployed. my husband's had an affair. now my attorney who i work for has fired me. yeah, pretty low. keith morrison: such problems. and then there was him. emmett corrigan fixed everything. he was handsome, and he thought she was gorgeous. and of course, he hired her right away for his new law office. and, well, you know what came next. soon there were racy emails, spicy text messages. i would like to be put on that pedestal. emmett made me feel that way. it was pretty much of an ego boost for me. keith morrison: and she really wasn't trying to get back at her husband, said kandi, at least not consciously. i was thinking about me and only me. you know what, it made me feel good. it made me feel like i was on top of the world. keith morrison: sitting here, now, is kandi still thinking only of kandi? perhaps as you hear the rest of the story, you can be the judge of that. anyway, back then, there were a few hitches in kandi's newfound fantasy life. to start with, emmett corrigan was also married and lived in this quiet cul-de-sac just a couple of miles from kandi with this woman, his wife, ashlee. he was just a guy that everybody wanted to be around. enthusiastic, full of energy. sometimes too much energy, but that's kind of one of the things i loved about him. keith morrison: just like kandi and rob, ashlee and emmett met in a gym. theirs was in college in utah, 2003. also inseparable from that moment on. and they certainly made a striking young couple. they were married after just six months together, made their vows before god and the church in the lds temple and-- ashlee corrigan: we both were like, you know what, i think i'm ready to be a parent. keith morrison: first came twin girls, followed soon by a son and then another daughter. ashlee corrigan: he loved being a dad just as much as i love being a mom. keith morrison: in the winter, they went skiing and sledding. in the summer, they camped and swam at the lake they so loved. they made memories. ashlee never doubted this was how their life was supposed to be. she was pregnant with her fifth child when her ambitious husband opened his law office that fall of 2010 and made the fateful decision to hire a paralegal named kandi hall, not that he had any idea he was sealing his fate, of course, any more than his wife ashlee understood his private motivations. did you suspect she was involved with him at all? with emmett? keith morrison: yeah. no. why? the way he described her was an older woman who he looked up to in a motherly way. he said, she just believes in me. she thinks i'm going to be this great lawyer. and you saw her, and she was an older woman. she was almost 40, and i was 28. so i don't-- it wasn't something that i felt like a competition of, i guess. keith morrison: but for the many reasons that plainly escaped those who aren't seated smack on the hot stove of desire themselves, emmett and kandi thought otherwise. oh, they tried to keep their hands off each other for a little while, said their co-workers. but if they believed they were hiding their obvious infatuation, their suddenly messy hair, their hastily rearranged clothes, they were only fooling themselves. i noticed a significant change in her attitude, went from being depressed about what rob had done to happy, you know, and-- a spring in her step again. oh, yeah. keith morrison: such timing. now that rob seemed to want to fix their marriage, kandi became a study in pretense. honesty took a holiday. i was living a lie. being in an affair is just living one big lie. keith morrison: yes, and she lied to herself too. you were thinking of you and he together striding across the bow of the "titanic." this is going to be it for you for ever. yes. keith morrison: then one night a couple of months into the affair, events suddenly ticked measurably toward their deadly conclusion. around bedtime, kandi received a text from emmett and couldn't hide it. the text popped up, and rob read it. and it said, "i wish i was there with you tonight." he-- - what did he say? he was angry. he just-- kandi, what is this? and i said, i don't know. so he calls emmett. rob goes, why are you texting my wife at this time of night? keith morrison: emmett's answer-- two minutes later, he showed up at rob's house. they talked like dueling lovers out on the sidewalk. then rob came back inside. rob tossed my phone up on our bed, and he said, you win. i can't compete. he's young. he's a good looking guy. he's an attorney. you make him a lot of money. what good am i? and he was just devastated. now you've got yourself a pretty complicated life at this stage. yeah. keith morrison: of course, that february 2011, emmett's life was complicated too. ashlee could see how stressed he was, didn't understand it, that or why he seemed to avoid coming home. there was one time when my son asked him if he lived with us anymore. and later in the bedroom, he was like, what's that all about? and i was just like, well, we miss you. and he just-- he just kind of yelled and screamed and left. keith morrison: ashlee thought maybe it was her fault. she went to marriage counseling. emmett refused to go. i had felt really pushed away and was trying to find an answer and try to surprise him by cleaning out his car and found a weird envelope. weird envelope? what-- just an envelope that was some sort of pill. i researched online, and one of the side effects was problems with intimacy and sexuality. i thought maybe if he was doing that that could explain why he didn't necessarily want me. did you take it personally too, though? you want to be everything that they want. it was hard not to take personal. keith morrison: especially when emmett, who'd been working out more and more, announced he was going to a fitness competition in ohio on their wedding anniversary. what did that feel like? lonely. he called and said, "happy anniversary, i'll call you later," but didn't call back. and the night before he came home, my oldest son, who was-- he'd just turned four-- was screaming one night for probably two hours. "my dad's going to die. my dad's going to die. he's going to die." and i just held him, tried calling him, but he never answered his phone. and it was a very strange weekend. keith morrison: strange? strange is not a big enough word to describe what was about to happen. craig melvin: coming up, a secret meeting at walgreens, and something will go horribly wrong. i said, no, we're not doing this. and he's like, oh, we are doing this. craig melvin: when "dateline" continues. (michael) copd is harder on the folks around you, i think. my wife smoked, and she quit. because she needs to be here for to take care of me. (announcer) the people you love are worth quitting for. you can quit. call 1-800-quit-now for help getting free medication. [ doorbell rings ] you must be isaac. come on in. call 1-800-quit-now for [ sighs ] here's my pride and joy. 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"i think she might be sleeping with this person. she says i'm the worst father ever." and just things that i know i had never done. so when he walked out, i flipped the baby monitor off, and i said, oh, how did it go? "it went good. he thinks you're as crazy as i do." and i said, "ok, well, do i get a turn?" he said, "i don't care what you do, but you're not using my phone." and emmett said, "hey, i'm going to run to walgreens, and i'll be right back." i put the phone down, and i literally was like, "emmett, please, do not leave." and he said, "no, i'm leaving." you must have felt like your life was flying apart and you didn't know what to do. i kind of felt like, ok, maybe this is the grand finale. but he needed a wakeup call. he did. he needed a wakeup call. careful what you wish for. getting late now-- very dark. over at the hall house, kandi had been talking to her husband, rob. maybe he shouldn't move out. maybe they should try to fix their marriage, make it work somehow. and then, right in the middle of that, she suddenly told him she had an errand to run, couldn't wait. guess where. i said, i need to go to walgreens. i'm just going to go through the drive-through. and i said, i'll be right back. keith morrison: here's kandi's explanation for the way the meeting with her lover was arranged. as i was pulling out of the back of my driveway, emmett texted me and said, "hey, what are you doing?" i said, "i'm going to walgreens." and he goes, "i was just there. hey, meet me there." keith morrison: and then, what happened next-- you can watch it yourself right here on surveillance tape. kandi hall: i go to walgreens, go through the drive-through, and then