Late in the game, Agent Milton tells Arthur that Micah turned rat after the gang returned from Guarma. However, I think Micah was a rat all along, in one way or another.
First was when the gang was ready to move from Horseshoe Lookout. Dutch sent Arthur and Charles to check out a new spot that Micah had recommended. It turned out to be a large meadow. Arthur and Charles immediately realized how out in the open and vulnerable it was. It would have been easy for the gang to be ambushed and surrounded.
Why would Micah recommend that meadow? A place so clearly wrong for hiding the gang? I think he was trying to set up the gang for capture, and that Cornwall was going to pay him. Keep in mind that Cornwall had just confronted Dutch in Valentine.
Micah’s plan was foiled by Charles and Arthur stumbling upon Clemens Point (when they rescued the German man) and deciding that was a better spot. Charles rode back to Dutch to suggest Clemens Point as their next camp instead.
Next, when Dutch met with Colm O'Driscoll to talk about ending their feud. Everyone was hoping it would lead to peace between the two gangs, but was suspicious that it was a trap. Sure enough, it was a trap… to capture Arthur. Colm's idea was to grab “Dutch’s son” Arthur in hopes of luring the entire gang out of hiding to rescue him. Colm was planning to turn in the entire gang to the Pinkertons in exchange for leniency for himself.
You may recall, Micah was really pushing hard for that "peace meeting" to happen. We all know Micah doesn't care about achieving peace or making friends. He’d normally vote for just killing everyone. So why was he hard-selling the peace meeting to Dutch? Because he was the one who set it up. It was Micah’s idea to bring Arthur as back-up. When Arthur, Dutch, and Micah went to the meeting, Micah told Arthur where to position himself as sniper cover. Arthur was then quickly taken by surprise and captured by O’Driscoll boys. I think Colm was going to pay Micah.
Micah’s plan here was foiled by the fact that Dutch unexpectedly did not send anyone to rescue Arthur. You may think that it was just a matter of timing, that Arthur freed himself before Dutch had a chance to form a rescue plan. I saw it as foreshadowing for when he would also not attempt to rescue John from prison, and later when he left Arthur for dead at the oil refinery. When Arthur returns to camp after escaping the O’Driscolls, his dialogue with Dutch sounds kinda like Dutch didn’t really have a plan to rescue him. Dutch was starting to (or continuing to) lose his mind and had no intention of rescuing Arthur. Micah and Colm didn't anticipate that.
Another thing to consider about Micah is how there are parts of the game where he is missing without explanation. I’m referring to the time after Arthur freed him from the jail in Strawberry. At the time, I assumed it was because he was simply away from camp kinda like how Arthur was sometimes, but none of the other characters are away from camp for large amounts of time (unless it's part of the main storyline).
I explored camp life at length, and Micah is absent at several key points. He’s most notably absent around the times that Cornwall ambushed Dutch in Valentine, when Jack was abducted, when the gang went to rescue Jack from Braithwaite Manor, and around the time Kieran was abducted. I think Micah had a hand in all of those crises.
Basically, Micah was attempting to sell out Dutch and the gang from the start, to whoever would pay him: The Valentine sheriff, Colm O’Driscoll, The Pinkertons, The Braithwaites, Angelo Bronte. Micah was kissing Dutch’s boots and whispering ideas to him in order to control him, with the end game being turning him in to the highest bidder.
TL;DR – Micah was a rat from the start, and there are several key moments to support that notion.
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