Magic Man
Reaper of Conversation
You have not explained how killing them is "revenge", yet locking them up for life is not revenge.
In fact, the only difference I can see is the death.
What makes death an act of revenge as opposed to "locking them up for life"?
I've covered this. They are a danger to society. We have to remove them from society to keep it safe. Locking them up accomplishes that. Anything harsher than locking them up is unnecessary, meaning capital punishment is unnecessary. That means you're taking a further step just because you think it's what they deserve. That's revenge.