Greed and loss of control.

All of the above, kinda 🫣

All of the above but I believe what Arthur said “he became more of who he really was” these things only contributed to revealing his true colours.

Dutch was a lying, manipulative, narcissistic psychopath. He was only nice to his gang, took care of them, & believed in them because he knew he could finesse them into always bringing in money for him & doing all his dirty work. I mean obviously there were a lot of times throughout the game where he did genuinely care about his gang & helped & looked after them. But at the end of the day, he was mostly just looking out for himself & his own ideals. Dude's a piece of shit. I mean hell, by the time chapters 1 & 2 happened, Hosea, John, & even Uncle already saw through his bullshit before anyone else did. That right there says it all. But obviously they stuck with him at the time because ya know, loyalty. Hosea was the only person that could keep him in check. Once he died, Dutch started fucking up, showing his true colors, & eventually let his own greed slowly kill him. Plus Micah manipulating him didn't help either obviously.

Never thought of the trolley injury. Good point. And then after that he was making imaginary chess moves

I think Hosea's death, although destabilising for his mental state, was too late to be a factor. I also think Micah wasn't the main reason (though, again, probably still contributed a bit), because Dutch remained how he was after splitting with Micah and took a liking to him right from the start anyway. The head injury in the train robbery is a fat red herring, in my opinion - there's no evidence it had any impact on his own given that he was already going mad and plenty of other things happened soon afterwards anyway that they can be used to explain his behaviour.

I think it was a result of years of intense yet ever-increasing stress and pressure. Being a very wanted outlaw with lots of people to protect, constantly having to be on look-out, couldn't trust people easily, etc. is bound to have worn him down and make him get more desperate as the West was slowly ebbing away. It seems like Colm also went mad to some extent given the erraticism of his decisions, incredible capacity for ultra-violence and inability to be reasoned with. Micah went mad by 1907 after being the baddest outlaw in the West for years, Bill couldn't handle the pressure of being a gang leader and also went mad by 1911. Seems that it happens to a lot of characters.

John doesn't attribute anything to Dutch's madness in RDR 1, implying that it was a slow process, which would certainly favour this explanation. He also talks about power a lot, claiming to Drew MacFarlane that it's (paraphrased:) "like a drink, the more you have, the more you want". He also remains suspicious of Reyes' desire for power over Mexico and clearly seems to have formed an overall opinion that people are corrupted by power and can't handle it. Given how much of a major figure Dutch was in his life, it's not implausible for John to have formed this opinion, among potentially many others, from watching his mentor.

Imo he doesn't go crazy, he just reveals who he truly is. He may legitimately care about the members of his gang, but he always cared about himself and his desires way more. An example of this can easily be seen by Dutch pursuing Mary-Beth while being with Molly.

I however do think Micah manipulating turned him against the people he was closest to like Arthur and John, and with Hosea being dead there was nothing to keep him on track.

Probably a combination of michas manipulation and the fact that he may have been alone for a very long time after michas death

I'm going with the trolley because before that he was more sane and not reckless like when they went after Bronte and when Dutch killed Cornwall