Why not drain the blood of the accused? It's far less painful than the reactions to some drug combos. (For those lefties here) It could benefit society and pay back the debt that is owed by donation. (Everybody wins) The death penalty is still usable, humane, and beneficial.
The problem, I think, with the death penalty is that once you hand the dysfunctional justice system an option, it often gets misused and overused. Yes, theoretically there are some people who deserve to die. I'm just concerned about the other ones.
Perhaps a heroin overdose. Absolutely painless, it is after all the most powerful painkiller known, and 100% reliable if the dosage is high enough.