As explained in the last entry of this Cracked article, the relationship between Munny and his deceased wife. There is nothing at all in the movie to explain why she fell in love with Munny, just that she did, and stuck through with him despite his dark past. And likewise, Munny must have seen something in her too; he was an outlaw from whom legends are made, with great ruthlessness and great skill—and he put it all down for her, to be a simple, struggling farmer.
In the end, William and his children go to San Francisco. William prospers in selling dry goods, finally being able to leave behind his violent past life and find redemption not as some avenging hero but as an honest businessman.