I agree with you that it would be morally justified in that case, but I don't think that any human institution can be trusted to do it. There's just too much at stake and too many ways it can go wrong that even if killing someone for a certain crime is completely morally justifiable there will always be an unacceptable margin of error.

It's understandable why the death penalty is so attractive to so many, but it should be seen as a relic of the past and left behind as much as capitalism itself.