“Does the death penalty deter people from committing horrendous crimes?”
Whether the death penalty deters someone else is not the point. The real question is, does someone who slaughters another person deserve to die or not? The answer is clearly yes.
You raise a good point here, from a philosophical and humanitarian standpoint.
I must confess one thing here, which is important. I am a Christian who does not believe in Hell. That is crucial in explaining my position.
A hypothetical ‘slaughterer’ who gets life without parole will be faced for the rest of his life with the consequences of his act(s). Which amounts to an endless and torturous repetition of the same, boring, and perhaps violent (in some jailhouses) day. Over and over again.
He who is executed will have a transit to... I don’t know. What happens next is a question of one’s religious beliefs.
But, let me criticize myself here: I know very well that those left behind, family and friends of a victim, deserve some form of consolation (I’m Dutch - perhaps this is not the right word, but you’ll know what I mean). And if that consolation is best given by making sure the murderer does not exist in bodily form anymore, then it’s a valid argument to administer the death penalty.
Feel free to criticize. One always learns from decent criticism.
It's the equivalent of discussing nuclear physics to a hound dog. It just won't make sense to them, not because they're stupid, but because it's that these concepts are not part of their psyche and ethos.