Maybe its just me, but if someone rapes an 8 year old girl, kills her, and then puts her in a suitcase at the bottom of a pond, their death should be as painful as possible.
What does such mindless vengeance actually accomplish? Would you sleep better at night knowing that not only did the state put someone to death, they tortured him in the process?
And keep in mind that we can never be 100% sure of any conviction. Just this week Texas posthumously overturned a conviction based on DNA evidence (he wasn't put to death; he "only" died in prison, but as long as you're being sentenced for a crime you didn't commit, it could just as easily be a crime that does merit the death penalty). This is a good argument for why we shouldn't have the death penalty at all - it's irreversible. But so long as we still do have the death penalty, what about the chance that innocent people are being tortured to death? Don't they at least deserve to go peacefully?