At the end of the movie 3:10 to Yuma, we see Ben killed his teammate Charlie after failing to save Dan. We have seen that Ben tried to cooperate with Dan during the second half of the movie. So, was Ben feeling weak towards Dan? It's very unnatural given the role he has portrayed in the movie. Ben is a cold-hearted mass murderer. How come has he changed? And why did he try to save Dan and kill Charlie? Am I missing something?
Ben could've escaped whenever he wanted, he wanted to ensure that Dan sees him off in the train to Yuma, The reason being he wants Dan to succeed in his endeavor because of the promise he made to his son and his wife and also he sympathizes with Dan on the fact that he used to be a soldier and his life changed after he sustained the injury in the war and wanted to give him the fulfillment of sending the notorious criminal to the jail.
So when Charlie kills him against his orders Ben gets angry on that issue and he no longer cares for the group and he kills them all and goes in the train simply to fulfill Dan once again and once the train departs he flees on his horse.