I may be the only one who feels this way, but his behavior was utterly irrational. Forget for a moment how United handled the situation, which was by all means flawed, but think for a moment how he handled it. He behaved like a disturbed, unreasonable, lunatic. At one point he was muttering, "Kill me! Kill me!" Do you really want that person on your flight? What would he have done if the plane hits turbulence, or if a fellow passenger made him angry?
Apparently, he was willing to die to stay on that flight. If his meeting in the morning was as important as he claimed, then why would he be willing to die? How is that rational? Wouldn't he miss the meeting if he was dead? The man is mentally unstable. Or maybe he had some mission oriented reason that he absolutely MUST be on only that very specific flight. Either case is scary, and it is good that he was removed. IMO