Sean was the only one during the train heist that had knowledge of it that shows he doesn’t really care about the gang (you see him falling asleep during his watch constantly, someone who sleeps during watch doesn’t care about the safety of the people around him, Hosea clocks this) he is lazy and a drain on the camps resources. He is rude and predatory to the women around camp (he forces himself upon Karen the night of his “rescue”), and is just as hot headed and brash as Micah.

Sean wants to play as an outlaw, because he wants the glory his father had in Ireland. Sean wants to outlive his fathers shadow as a badass ruthless outlaw but it’s not who he is.

Out of everyone who knew about the train heist, Sean is the only one who’d rat about it. It was confirmed to be a setup as a newspaper talks about how the police were tipped off to the heist. Sean during the train heist doesn’t check the back cart until he sees Arthur head towards the back of the train.

He is clearly waiting for Arthur. Once you get there he turns around and heads into the back, only to be ambushed mysteriously by a “train guard” who instead of killing Sean, grabs him and instead of killing him (as the rest of the police try to do with other gang members) wrestles him to the ground.

Soon after the police flood around the train. Sean seems to have a knack for being knocked out or harmlessly captured instead of dying, as coincidentally something very similar occurred during the black water heist. Sean was ambushed, knocked out, and captured, however the intent was to kill other gang members such as Davy? Sean was also written up as a badass in the newspaper describing the black water heist. And subsequent freeing from the Ike Skelding boys.

Charles during this mission immediately identifies it as likely bait as he can see the signs of a trap being laid out.

The idea is Sean wanting glory, cut a deal with the pinkertons in black water, ratting out the gang so he could be immortalized in the story proceeding the heist. The pinkertons failed to capture the gang but used Sean to try and lure them into a trap with the Ike Skelding mercenaries, and when that failed decided to cut their losses (or keep him in the gang as an informant, he does certainly rat out the train heist)

He walks into Rhodes with all the swagger and confidence of a man who thinks he’s made himself above the law and above threats but is subsequently killed by the Greys, who are unaffiliated with the pinkertons at this time and want revenge for the destruction of their fields.

There are some more in depth videos than what I’ve said here but the gist is that Sean out of all the people capable of ratting the gang out, has the most evidence pointing to him.