You know............there IS a painless, humane way to put people to death.
It's called helium. Helium gas is VERY dangerous, because you can suffocate if you breathe in too much of it. Best way to do the execution? Put the person under with nitrous oxide, and when they go to sleep, replace the nitrous with helium, and they'll be dead within 5 minutes.
Matter of fact, you could even put a small chamber around a chair and pump it in via the floor.
They don't need to use injections that don't work.
Besides.............there are suicide kits that you can buy on the 'net that is nothing more than a bag with a tube that connects to a helium bottle, you have to provide your own helium.
As far as seppuku like someone else mentioned farther up the thread? Nope, that is a ritual suicide that bestows the person doing it with much honor. In the days of the Samurai, when they had done something to bring dishonor to themselves, they were allowed the chance to do seppuku as a way to regain their honor and die like a Samurai. If they didn't want to kill themselves, there was another Samurai behind them, ready to chop off their head if they hesitated.