I don't support the death penalty, and never have.
I think it brutalises everyone involved, from prison staff to the people making the implements of execution. Imagine going to work knowing that you are going to make gas to kill someone, or rope to hang them, or to measure someone up for a gallows or electric chair - it's horrible, and gives far too much power to the state.
There is also the risk of getting it wrong, as discussed.
I don't think that any threat of punishment would stop the likes of Brady and Hindley. They are insane/evil/ill depending on your point of view, and no deterrent would work.