Milton did his job and gave the Arthur John and other plenty of chances and never resorted to underhanded tactics like Ross did despite everything Milton is law man and holds himself to that standard
Miltons mistake came from trying to convince the hardliners like Dutch and possibly Arthur although Arthur was starting to have doubts and stupidly evil like Micah
He should tried John and Abigail who were quite dissatisfied with gang and Dutch’s breakdown
Ross sadly is no better than the outlaws he hunts his tactics would disgust Milton especially using and betraying a man who learned and reformed seeing the true vileness of crime hell he disgusts his fellow agents and soldier of US army during red dead after John died no one liked him other than his family