Movie's message - When is killing someone ever OK?
I think the movie's message centers on whether murder is ever clear black-and-white, good-or-bad. Is it possible to make an accurate determination of when killing is correct, and if you cannot, then should you ever kill?
Let's go through the movie's major killings:
NED --- Instinctively, you'd say that it was wrong to kill Ned, as Ned didn't kill Davey and was heading home. But I could argue that Ned had it coming because he did ride with Munny and the Kid; he did help them locate the town and Davey; and he did fire one shot at Davey (though he refrained from the fatal blow). Ned therefore was an accomplice to a crime, even if he left. Little Bill had every right to question Ned, including by force. Little Bill inadvertently used too much force, of course, but he wasn't totally unjustified.
LITLLE BILL --- Some might say Bill deserved to get killed because he killed Ned, the innocent one. But as the girl noted, Bill had no intention of killing Ned. He killed by accident, and he did so specifically because he wanted to keep murder, bounties, and lawlessness out of his town. And though the viewer knows that Ned didn't kill Davey and was heading home, Little Bill had no way of knowing that. From Little Bill's POV, Ned was an accomplice to murder.
DAVEY --- He was the one who held back Quick Mike when Quick Mike cut the girl's face. Davey was the one who acquiesced with Bill's demands to give up their horses. Davey was the one who also sought to make amends with the girls, as he tried to give her a pony. Davey 100% did not deserve to die. Yet he did die, killed by our protagonists. All they saw was a guy who had a bounty on his head because he hung out with Quick Mike. They didn't see Davey as the good person that he really was.
QUICK MIKE --- Instinctively, you'd say Quick Mike deserved to die. But did he? What exactly did he do? He cut a girl's face, which is really bad; but is murder the right punishment? By law, Little Bill already exacted punishment in the form of their horses. By law, Quick Mike paid for his crime and was done. So did he deserve to die? Let's also remember the story that the Kid gave -- he said Quick Mike cut that girl's face, cut her eyes out, cut her tits. And that's false. He didn't do all that. As confirmation that Quick Mike didn't deserve the die, the Kid is traumatized after the killing, and the kid resolves never to kill again. If Quick Mike deserved to die, then why this guilt?