If we want to kill people for doing something bad, then why are we concerned about doing it humanely? That doesn't make sense. Killing them is not humane. Killing them is about our wanting to kill them, so why not go all out? Draw and quarter them. Boil them in oil. Burn them at the stake with green wood which burns more slowly. Infect them with a flesh-eating disease, and so forth.
Make it as horrible and as awful as it can be, since it's for our benefit anyway, to taste vengeance on our tongues and satisfy our craving for sanctioned murder. Why mask it as being humane, when the very act itself however its done is not humane.