[crusso]

Regardless of how horrific their crimes were we can never really be certain enough of their guilt to kill them

That's a strange thing to say categorically. So if we have a lot of witnesses, video, a manifesto of motive, physical evidence, etc... we can't be certain enough to feel that someone should be permanently removed from the world?

[k-mcgrady]

IMO no. Witnesses can lie, video can be manipulated, physical evidence can be tampered with. Obviously we can be very close to certain but unless we are 100% certain (which I don't think is possible) killing someone is a big risk.