The reason why Dutch killed Micah isn’t clear after being together for so long and at the end of the video game. It is after John appearing and having a little conversation that Dutch pulls the gun and kills Micah.
Has stress level gone through the roof after all the fights? It is madness and it is obvious that Dutch is incredibly crazy. He loses his mind and starts making actions that seem crazy and dangerous throughout the game.
Some have even started to diagnose Dutch with a plethora of mental illnesses in order to better understand why he acts the way he acts. One of which that stands out is bipolar disorder that is triggered due to the high-stress nature of the gang’s final days and of one particular incident.
Well, one idea I’ve heard be thrown around is that in his old age, in seeing John there with the intent of killing Micah. In thinking back of what happened so many years ago, he sees the events with clarity, and realizes it was indeed Micah who was the rat. And maybe this train of thought makes him wonder if Micah again is leading him down this path where Federal Agents are waiting for him.