Apologies if someone else already pointed this out, but the boy wasn't being forced to watch his 'prostitute mom' have sex... that was the family with the boy that the Cowboy saw earlier when he was told he would have to wait till the next morning for his medicine. The men in the back room of the bar had robbed the family, killed the father (who was there dead on the floor) and were raping the mother while forcing the boy to watch. Storywise, this was setting the Cowboy up to have to make the decision he does to return when he is riding out of town and sees the new family coming in to Ratwater. He took no action for the first family's destruction, and plans to take no action to save the second family... but changes his mind. That's why he goes back, and he punches the preacher because he is trying to get to the backroom to save the new family he knows is there. That's when he sees the family turning in scalps, not getting robbed and murdered, and then gets knocked out himself. The horse was not just killed as lazy writing to make the preacher "bad"... it was killed so that the Cowboy would have to walk all the way home and thus, be too late to save his family. It was all set up to make the Cowboy return to what we assume is his once-set-aside past of being a total badass. I expect he will be returning to Ratwater soon now... with all his rage and all those weapons.
