At some point this mentally suspect individual was given a choice. Get off the plane or you will be removed. His injuries are the result of his choices. He obviously had never been taught to share as a kid and thought that his belligerence would be rewarded (as it obviously has been before). He might think twice in the future.
He doesn't have a legal right to stay on the aircraft. He was told to leave. It's pretty simple. Obviously the
PR issues are significant but in the end you just have to be part of society and sometimes that means you don't always get what you want. Individuals need to be adults. This guy was just throwing a tantrum.