An alternate perspective: these people have done something that arguably makes them not worth rehabilitating. They are willing to literally kill the citizens they're supposed to protect, seemingly for no reason at all other than perverse enjoyment. A man is dead so that these people could exercise their authority and power.

I personally don't support the death penalty... largely because I don't trust that the government will use it correctly, and that largely based on decades of evidence of wrongful convictions.

But if it were to be allowed and used correctly, it would have to be a method of removing un-rehabilitatable, dangerous people from society. People who either cannot ever be trusted not to be a danger or who have shown that they are unwilling to cooperate with reform attempts.

I don't disagree with the death penalty as a concept, in some fictional ideal world where it would only ever be used on people we know will never not be dangerous to society. We don't live in that world, but I sympathize with people who believe that people who commit crimes like this should just be removed.