It's tempting to think people like this are stupid, but not necessarily. You see this sort of behaviour on forums at times, even from people that very obviously aren't stupid.
He's got something wrong, but can't countenance the idea of being wrong. So he digs himself in deeper and deeper, and subconsciously redirects possible guilt at being wrong into anger at anyone that points it out, getting more and more abusive to anyone around.
It's a character defect that we probably all have to some extent and is nothing to do with intelligence. How much easier to say "Oh, sorry, I got it wrong" early on, than later, having invested so much personal credibility into the wrong idea. At that later point, any rational reassessment of one's ideas becomes very difficult.