That's a good point.
But, going the other way, even D&D Demons and Devils have some (albeit small, but some) chance of being redeemed.
So, from your perspective, when is violent action acceptable?
Depends entirely on the situation. "Clear and present danger," so to speak. The likelihood of redemption. A currently-violent/dangerous fiend
can be redeemed, but the chances are so crushingly slim that, barring the PCs having extraordinary powers at their disposal, it's usually not worth it to try. After all, it's entirely logical to say that fiends can't be redeemed through simply being kind to them and you need to use heavy-duty magic to even give it a shot.
Ditto, an orc raider. Can you capture them safely? Do you have the means to hold the orc while you try to show it a better way? Do you have a reason why it might believe you? Is the orc themself willing to put down their arms when you believe them? And most importantly, if you focus on
this orc raider, will the
rest of their band then destroy the town?
That being said, I
did once run a game (GURPS Dungeon Fantasy, so not quite D&D but close enough) where the PCs decided to start a workshop in order to give goblins and kobolds and the like a job other than banditry. And in a much more recent game, I had a spectator attack the party (as it was being a guard), gave it the ability to summon
another spectator (because the spectator would be too-low a CR otherwise), and when the party killed the first spectator, the summoned one asked "Uh, why are we fighting here?" because he had better things to do than get into combat with the PCs and didn't care if the PCs took all the stuff that the first one was guarding. Combat was dragging a bit at this point and I gave the PCs an opening for negotiations--and they took it. The spectator left peacefully.