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.
How about euthanasia?
It’s a weird edge case that is between killing and assisted suicide, assuming there is consent.
Euthanasia without consent seems not so humane to me, but once again it is an edge case that is still hard to define.