when he murdered, dismembered, and burned the corpse of a fellow high school student

How can someone even go on living with something like that? You can't do something like that and then just go on living as if nothing had happened. That's not what the human psyche is made for. Sometimes things escalate, sometimes they escalate so badly that someone dies, but you regret it afterwards. Only a true sexual sadist monster can probably go on living with the flashbacks to a cruel crime like this without his heart rotting away because it's already rotten, I guess?