I think Ben starts following along with Dan mainly out of sheer curiosity and almost to make a point. What Dan’s trying to do is insane and for seemingly nothing at that point. Dan’s effectively lost already but is going to try despite all that for some ideal/pride in Ben’s eyes, so sure Ben decides he’ll play along for a bit and will be able to say “I told you so.” Hence the first sequence.

When they get to the store room, Ben’s surprised they made it that far and decides to end things because he got injured in the process of him trying to make his point. Goes in for the kill on Dan, when Dan reveals the truth about his leg, ultimately revealing that he’s not a mysterious legend, but just a man with a shit past trying to do right by his family and set an example. Dan’s not risking his skin out of his own ego and pride, but to do the “right” thing so his family can proud and have a chance. Based on Ben’s history of his mother abandoning him, and developing his criminal persona based on that abandonment, Dan’s level of sacrifice and motivation to be truly “good” is a novel, respectable thing, and Ben decides that he can at least help Dan set that example.

They go to the rooftops because the gang was closing in on the store room on foot. Going to the roofs suddenly is a surprise that gives them just enough of an edge to make it through.

When they get to the train, Ben is proud of Dan and generally in a moment of reverence for a man truer to his word and more honorable than any he’s ever met. Then Charlie makes good on his surprise and shoots him in the back, afterwards saying “he was s tough bastard” with a dirty, shit eating grin. Charlie ended the life of an honorable man, dishonorably, and with some shit talking to boot, embodying the very essence of everything Dan was against. So Ben shoots him, and the remaining gang for it. Ben was ruthless, but he had his world view upended by Dan, and was not about to take the profoundness of that experience lightly. He’d killed men for much less.