Why did Ben try to help Dan?

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.

I think Ben was was likely to be good, but since his mother left him while reading bible, in childhood, he was left to face the odds of society, that made him become what he became... but there were two things left in him of that memory, bible and respect for family. He killed one bounty hunter when he told him he was son of a wxhore... In whole movie he was reading proverbs and references...

In the movie he was shown drawing things he liked, an Eagle, a woman, ... and DAN! He had liking in him about Dan, because he didn't gave up on the family's pride, even when Ben taunted him of things he should've done better for his wife, his sons... Ben couldn't break him... he mentioned that part of Dan he liked while fight in mountains... and when in the station Contention, Dan just told him of why he wasn't stubborn... this was turning point that made Ben believe that this guy has guts not to leave the family against all odds... and when they left the station for train, they nodded each other of a plan to enable Dan hand over Ben to train's staff...

well at that ending part, Ben was living in retrospect ... thinking about any good left in him was giving Dan a chance to succeed with his family's issues, not just a few bucks, but win his respect and he admired his struggle for his family that's why he gave himself in. But it was only among these two... Ben's crew didn't had glimpse of what's going through his head... and when they realized, it was too late for them...

Ben used to trust God, that's why he read the Bible from cover to cover hoping that God would make his mother come back. When she didn't come back Ben stopped believing in God, and if God doesn't exist for you then you can do anything you want, but deep inside he still had that feeling of righteousness. You could see that when he tells stories of Apaches being massacred.

The more Ben interacted with Dan the more Dan grew on him. He liked Dan. Now, why did he like him? Because in Dan Ben saw himself, the other himself, the other himself he probably dreamed of being but couldn't be - a righteous man with a wife and kids, who had principles and who would die for his family and that's what Ben admired in Dan.

And the last reason is that Ben knew he could escape Yuma otherwise he wouldn't be that helpful.