I once had an argument about the death penalty in college and my opponent said "What about a monster like John Wayne Gacy?" and indeed I have no feelings about such a man.
But, in killing John Wayne Gacy we also made him silent. He was a monster who killed many kids and young men. He took advantage of the friendless and frightened.
But, to this day people wonder if he acted alone.
Were he still alive we might ask.
Keep even the worst alive, it is preserving evidence.
@futurebird Christians probably believe they send evil people to hell by executing them. But what if the atheists are right, there is no afterlife, you simply cease to exist when you die? In that case, death is far too good for monsters like that. They should be kept alive for as long as possible, but in a tiny box all alone.