monta, you've stated several times that you don't understand why people support the death penalty. So, I'll try to explain my feelings without getting all worked up.
Maybe it's an effective deterrent and maybe it isn't. I don't know, and tbh, I care very little. It strikes me that prison isn't much of a deterrent, either. At any rate, it serves at least one practical purpose: threat of the death penalty often spurs criminals to accept plea bargains of life without possibility of parole. This spares the state the expense of prosecution, and more importantly, spares the victims/ families the emotional roller coaster of a lengthy trial and the possibility (however slim) of a monster going free.
But it's not about that. It's about drawing a line in the sand. About taking a stand on things that we simply will. not. tolerate. That some things are simply unforgiveable. It's not so much about the darkness as how far we're willing to go to fight it.
I often wonder, though, how heavily my stance on this is influenced by other factors. I have no fear of death, and indeed, would prefer it to incarceration. Then there's the theological angle: I believe that the "souls" of such people are suffering, and that death will release them. I also have an admittedly rather primitive understanding of justice: an eye for an eye, a tooth for a tooth, a life for a life. UU spurs me to seek a better way, but when the ideal falls to ugly reality, my instincts are rather harsh. And finally, this is all very personal to me. As you may recall, I was horrifically abused as a child. I've made my own kind of peace with that, and I'm obviously not unbiased, but I truly believe that my abusers are beyond redemption, and the only way to stop them is death.
Put all that together, and perhaps I accept the idea of capital punishment a little too readily, but that's where I stand.