Seems to really stretch the term “humane” thin. There are a dozen simple examples that poke holes in this wide net. What about killing someone who is willing to die trying kill you? What about someone currently on a suicidal killing spree (as happens once a day at a school)? What about the millions of animals raised for slaughter everyday? All that is the same category as a painless dispatch and a torturous murder?

Being humane is about how you do something given the choice. It has nothing to do about the absolute action.

Please read Consider the Lobster.

https://faculty.etsu.edu/odonnell/readings/lobster_dfwallace.pdf