The method makes a hell of a difference.
I'm personally undecided on the death penalty. A life sentence makes a better penalty but it's fucking expensive and I don't really want to pay for it, not sure whether the deterrence value is any better either. So there are some good financial arguments for putting down the obviously guilty beyond all doubt. Keeping them alive is like a really expensive form of hoarding. That doesn't mean you make the death as slow and painful as possible, and the only people who'd volunteer for that are usually criminally-minded themselves.