Steve,
The problem is he BECAME a suspect when he refused to exit the plane. He became of suspect of at least 2 offenses -- trespass, and being disorderly conduct on a plane. He was offered enticements to leave the plane, and refused them. The officers warned him that if he didn't leave voluntarily, he would be removed forcibly. How long were they supposed to talk to him and try to wheedle him into leaving voluntarily? It appears the air crew members that he was losing his seat to were being moved to maintain schedules; delaying that particular plane would have disrupted the schedule of every plane along it's flight line, as well as the ones similarly impacted by the flight crew. And the planes coming into that terminal, and at each terminal along the way... Huge ripples. How long were they supposed to take?
Were there better ways to handle that incident? Hell, yes. Way before he was yanked off the plane, United had options. Even once boarded, they had other options. But he contributed to the situation, too, with his behavior. But, once the cops are involved, at some point -- he IS going to be removed from the plane. And real world use of force is not pretty.
Dirty cops are pretty rare in US, despite the press and noise from groups like BLM. And the majority of those who are don't deliver unwarranted beat downs; they're on the take in some form or another. The odds of you being in a situation with a cop who is going to deliberately harm you are very low. The best way to avoid that small risk as well as the risk of dealing with the more likely "everyday corruption?" DON'T BREAK THE LAW. Amazing how few potentially hostile encounters you'll have with the cops if you don't break the law. And avoid places where other people routinely break the law. Throw in "don't be an *******" and you'll prevent most problems, whether dealing with cops or rude clerks...