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.
The key metric for ‘human’ actions is to avoid needless suffering. Livestock processed humanly is not panicked, not given a slow painful death, it should be quick, clean, and without pain.
Living conditions are human if they don’t create needless suffering, i.e. animals living in a open farm are human, animals living their lives in changes is inhumane.
I think the golden rule applies, treat others how you would like to be treated, is basically being humane.
What if you are a masochist?