Minutes later, he was back on the plane.
They once again told him he would have to disembark.
He refused.
They called the airport police.
He was removed.
The version I heard was that they called the police to get him off the plane the first time and that after he had been removed he re-appeared on the plane, bleeding.
Yes, the guy has a sordid background, but I'm not sure that is going to help United in this situation because it would look just as bad if had they violently removed someone with a non-sordid background.
There is some interesting reading on this web page.
https://www.quora.com/Is-it-a-crime-to-disobey-a-flight-attendant