A person can be killed instantly through many different means.
As we know it, to be killed painlessly, is to be killed instantly.
If you instantly destroy a person's entire brain then they died without pain.
Yet the most effective ways of instantly destroying someone's entire brain are considered inhumane.
If instantly killing someone by smashing their head with a massive rock, or shooting them in the brain stem is inhumane, then there is no humane way to kill someone at all.
And (afaik) with the executors mental health in mind as well.
e.g. if you just crush a head with a stone you will know you did it.
If two others and you have to push a button you can tell yourself that it’s more probable you didn’t kill somebody than that you did.