Hirst said that for a while he did not know when exactly Heahmund was going to die.
He knew there would be a huge battle at the end of season five and someone significant would die, but he did not know who.
Heahmund became the subject after it was decided his character could not have gone on any further afterwards.
Some fans hit out after his death, saying they were expecting bigger things from the character.
One said on Reddit: "I was hoping to see much more of him and redeem his character and make him a thorn in Ivar's side, but instead the writer's decided to do away with him entirely and not meet his full potential as a character. It's a damn shame."