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.
I posted it there as well but people tend to be very touchy about human death regardless of circumstances.
I for one don’t care what condition my body is in after I die as long as physically I didn’t feel anything.
That makes me think of the Klingons from Star Trek “It is only an empty shell; do with it as you will.”