One word: MONEY

He was just Micah, I dont think he had a motive other then being evil.

He was a mercenary before saving Dutch's life.

“I believe there’s winners and losers and nothing else besides.”- Micah Bell He wanted to win

Micah wanted to be better than everyone. He even states that before shooting Arthur on the mountain in on of the endings.

Because we are in different perspectives, in the eyes of the players, Arthur is a good person and Micah is a bad person, but in Micah’s eyes, Arthur is not a good person. Arthur is actually no different from Micah. To some extent Said that Maika is a very bad person

I see

Money was always my guess. Although it might seem that he realized that all his life he was so brutal and so terrible, that he thought he needed to do some right, maybe not by Dutch, maybe not even by god, but he needed to do right by someone.

Micah bell allways wanted and enjoyed Being a trecherous rat and snake

Personally, I think that his motives go deeper. I think that Micah wanted to destroy the gang completely in order to turn in Dutch and to get his hands on the Blackwater money.

I mean, think about it. I found a old wanted poster of Dutch in the Big Valley camp where Micah stayed at, so it's possible that Micah knew about Dutch before meeting him.

Canonically, Micah would need to destroy the gang by making a deal with the Pinkertons in order to kill Arthur, John, and the rest of the gang in order to save his own tail and to get his paws on the Blackwater money in order to escape the Pinkertons and to continue his life of crime.

Just watching his interactions with the other members, I think Micah betrayed the gang out of anger due to the fact that most of them (except for Dutch) hated him for his personality traits. It makes sense if Micah feels some way and wanted to get rid of everyone.

All and all, I figured out that Micah only joined the gang and saved Dutch's life was the start of his plan and combined with the suicidal ferry heist in Blackwater, Micah wanted to destroy the gang only to collect the money for his own benefit, but was forced to go through difficult tasks one-by-one in order to get the money, even if it meant killing Arthur, Dutch, and John to get his precious cash.