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.