His death would make his mission accomplished in his mind. But it is my belief that he will not find what he thinks in that instant between life and death. I think he finds nothing or he finds an angry God. I'm pretty sure there won't be virgins. So I don't mind at all giving him what he thinks he wants and letting him discover he made a big mistake - or not discover anything at all in eternal nothingness.
I'm not a big death penalty proponent either but this is a special circumstance as is any mass murder. The problem, I think and I'm no psychiatrist, with mass murderers, child pornographers, and religious or political terrorists is that their desire to do their evil is so deep in them that they just don't care; there's no conscious or empathy with their victims. Therefore, there's very little punishment in prison other than physical constraints.
What I like about life sentences is so convicts can live in their guilt and, at a bare minimum, the regret of having ruined their own life. With some types, like those listed above, that guilt never exists. They would spend their life in prison either daydreaming about what was or planning what will be again should they ever get released - sort of my obsessive daydreaming about winning millions in the lottery except that they actually commit their evil and I never remember to stop for a lottery ticket.
I'm not willing to pay for a life of pleasant day dreams of evil. Put evil to death. Save prison for those to whom it would be real self torture.