I don't know if he had a chip on his shoulder. He just talks like a troll on slashdot or an old BBS board, dropping in with a few paragraphs with a fairly strident position, and says anyone who disagrees with him is stupid.
I think people got super upset because he used his real name, he's a minor celebrity, and it wasn't a BBS forum.
I think he did it because it got attention, and he didn't really care about upsetting a few people on an internet forum. I also find it a bit disingenuous when people talk about how evil and deranged he is like they've never ever seen anything a bit strident posted on the internet.
Yes, there's some correlation between internet trolling and socioeconomic tendencies. He might be the kind of guy who takes milk from the work fridge and never tosses a quarter into the tin. But the people who seem most strident about how he's a terrible person are playing the same game as him.
As far as I can tell, one of his bigger controversies were essentially roasting a certain programming community for being smug and elitist (while being smug and elitist himself, along with a lot of swearing and insults). I wonder if the huge amount of offense caused (and taken) was partly because smug and elitist people don't like to be roasted like that, especially by a guy who is mostly famous for writing introduction to emacs tutorials (thus not exactly a technical elite, in a lot of people's eyes).