Arthur often finds himself being back-up, although more often than not it's involuntary. One of those faces, maybe- or bad luck, or maybe he pissed someone off with more power than him. Likely the latter, knowing him. The point is that he hears gunfire, and while he could tell himself don't get yourself involved, Morgan, he sees a woman just below him, exchanging fire.

He can see it's lawmen. And he knows most of the time, the law don't show up for peanuts. But the part of him that would help out a woman in need is about as big a his own contempt for those lawmen that do show up for peanuts. What are the odds that this woman, alone, deserves all of this trouble?

Fuck it. He wouldn't be able to sleep tonight if he just rode on by. So he stays near his horse, takes out his long-distance rifle, and shoots the first lawman he can confidently aim at. The bullet hits the man in the shoulder, and he goes down.