There are two things you can be certain of when you are about destroy a human life in the name of justice:
1. Someone is already dead because the death penalty did not act as a deterrant
2. Someone is about to die because the death penalty did not act as a deterrant
The one thing you can never be certain of though, is whether they are truly guilty.
The question is, are they really human? I would posit that somebody who can, in cold blood, murder 77 people, is not a human being. Murder of one person who has wronged you, or murder in the heat of passion is very human. But mass murder is not.
So, if someone is accused of mass murder, and there is a preponderance of proof, then that person is no longer a human being, and should be put down the same way you put down a rabid animal.