I'm guessing they made it painless because of the prohibition on "cruel and unusual" punishment. Regardless of the pain-free methodology, I'm not in favor of the death penalty for several reasons. Some of them are religious and thus have no place in the public sphere, but if any innocent person is ever executed then the penalty is worth discarding, IMO. That's a risk I don't believe we should take. An innocent person can always be released from prison but you can't very well bring them back from beyond the grave.