Given that either the police were acting illegally or United must have falsely asserted facts that made the removal legal, it seems to me that it must have become criminal, either on the part of the police or, more likely, United's false police report, before any struggle.

I think it's still up in the air as to if he had any kind of right to his seat.

United asserts, they prioritise their employees gaining seats on the flight because their employees are actually necessary to the operation of this flight and others.

It makes business sense, and its logical.

There's no law on any book that will force someone to serve you, whether it be transportation wise or by letting you have use of their mobile property (land/residences are a special case). United can pick random people at any time to get off the flight, for any reason, and at worst it will be a contractural breach or a violation of a civil statue (like the overbooking clause).

Once he was told to get off, he should have gotten off.

Absolutely not. In order for United to exercise THEIR civil rights in this entirely civil case and sue for lost revenue related to not being able to get their crew to Louisville, they should have let him fly after confirming his refusal to leave and then later pleaded their case before a judge. If United indeed had the right to reject him, they would have been compensated by the doctor.

It was United, not the passenger, who escalated to violence in a purely civil matter. He did not force himself onto the plane, he simply declined to give up the seat he had already been given.