I see that a couple of proponents of violence applied to passengers now try to justify the actions because the passenger concerned has a "record", only since uncovered? Are these proponents saying that he therefore deserved everything that happened to him? Are they saying that his sentences, whatever they may have been, were insufficient punishment and in their opinions, were not spent? I doubt any court of law would agree.
Strange that UAL were quite happy in the first instance to take his money, in advance, for a flight ticket.
His alleged belligerence appears only to have occurred once he was about to be forcibly removed from the aircraft. No doubt had his request not to "volunteer" to defer his ticket been taken notice of, he would have sat totally quietly.
To try to make out that he was going to compromise the safety of the aircraft is, quite frankly, ridiculous.