Agent Milton actually gave the gang a lot of chances, just a man doing his job at the end of the day.

Who was the Agent, Jack killed there's the answer

They’re both as bad as each other

Ross is a genuinely bad person who happened to work on the side of the law. Milton for all his theatrics was just doing his job

Milton is doing his job, he just has an abrasive personality. The most immoral things I remember him doing is executing Hosea (a wanted criminal) shooting a machine gun into the huts at Lakay, and taking Sadie as a human shield when she and Arthur assault Van Horn to rescue Abigail, which isn't much compared to Ross.

Ross kidnaps a family to force one man to act as a government vigilante, lies to him, then double crosses and guns him down. Milton was a private detective hired by Leviticus Cornwall, but in 1911 Ross is working with Nate Johns, a candidate for governor, to boost Johns' political clout by taking out notable criminals in the area. At this point Ross already has his position in the Bureau of Investigation, and to me he does not seem like someone who works for free.

^ a lot of that is incorrect.

Bureau of Investigation was founded after the events of 1907, Edgar Ross didn't have any position there until then.

Agent Milton offered the gang many chances, but who's to say any of those were genuine? I sincerely doubt he'd be willing to let Arthur and Hosea actually go free, considering they did as much as Dutch, Arthur arguably more than Dutch ever did.

He showed who he really was at Lakay..he knew there were many unarmed women and a child in the company of the gang, and he opens up with a Maxim like he's Colm O'Driscoll.

Honestly, even if he did let Arthur and Hosea go (and the rest of em), that would absolutely make him worse than Ross, because he cares more about Dutch than the other arguably much more dangerous members of the gang.

Holding a man's wife and son hostage is worse than anything Milton ever did

That man, and his wife and kid, were known associates of a gang that had committed multiple atrocities. John blew his chance at peace and quiet when he killed Micah.