Quote Originally Posted by JoeJ View Post
But on the gripping hand, if I take Benjamin prisoner and turn him over to the authorities, he'll be hanged because, in a medieval society, that's what happens to bandits who are caught. Probably in public, in front of a cheering crowd. So I'm not really sparing his life, I'm just giving him a few more days before he dies.
Because obviously powerful PCs have zero influence with local leaders, no ability to plead for clemency no matter how few Benjamin's actual crimes, and death is the only possible outcome for getting mixed up with the bandits so you might as well execute him now. Why even bother with fact-finding and a trial? Obviously there's nothing else you could possibly do.

Surely you can see that setting yourself up as judge, jury, and execution for a human being is at the very least morally complicated compared to killing a monster like a black pudding, a bone naga, or a mind flayer. The utility of monster-monsters is that they are morally simpler for when the DM wants to keep things simple.