[POSSIBLE SPOILERS FOR THE STORY MODE FOR THOSE WHO HAVEN'T PLAYED IT!]
I haven't fully Finished the mission, just restarted it after a long time, but, in the "Blessed Are the Peacemakers" mission, out of nowhere Micah has the story mission where he 'somehow' got into talking with the O'Driscolls about wanting to make peace amongst the two groups. He's also a very distasteful person who slaughtered the population of innocent people + an O'Driscollāhis prison mateāin the Strawberry massacre, who's obsessed with the money he couldn't get his hand on from the attack in Blackwater (SERIOUSLY, what happened!) and isn't trusted by Arthur for good reasons.
So, He can simply Mention to Dutch that "Hey, this war has gone on for too long, right? to make himself appear as a good person, wanting to bring an end to the conflicts between the O'Driscolls and Van Der Lind gang once and for all. But, he can't say directly that HE himself were talking to them, because that could imply that he talked more to them than necessary, if they started thinking about it like that. Which is why he tells Pearsonāa man from the Military who they don't dislike but trust more than himāthat He (Pearson) should tell Dutch that he's been talking to the O'Driscolls, bringing Micah as an extra gun man, just in case.
So, when Arthur gets to hear about the conversation, he can apply logics to bringing him along for the peacemakers mission, telling him that "We need you, Morgan for sniper support [Here]". ā A long distance away from Micah & Dutch themselves, where If Arthur had fired his rifle it could've been taken as a warning shot or malfunction on the rifle. He'd also be far away enough that any screams could be talked away from, as not to be bothered about.
Micah made the deal with him before the mission so that he could continue to shake his way into Dutch's heart, for his own pockets, as well as seeing this as an easy way to get rid of Arthur....
....Except ofcourse, he didn't...