My guess would be that his complete turn into assholery stems from a resentment of his life not panning out the way he wanted it to, or possibly what he believes he was told it would be. We've speculated at length about the possibility of Bob having a sheltered upbringing, where he was told (or absorbed from pop culture, more likely) things like "don't worry about the bullies, they'll be working for you one day," which warped his worldview into believing that he'd become successful just by being a nerd. By his own admission, of course, he was never the academically-successful sort of nerd that might have given him an advantage in life, instead being just a prototypical consoomer.
Funnily enough, Bob did have some early success by breaking into Youtube early. His first video was posted back in May 2006, barely a year after the site launched, and the Game Over
EaterThinker started a couple years later. The idea of being an internet critic was new, and since it was still a small pond, Bob could stand out and build a following just because there weren't many other options. Standards were also considerably lower, so stringing still images together with rudimentary effects (at best) was acceptable for standard audiences.
But that was over a decade ago. As more and more people decided to try their hand at launching their own criticism channels, viewers had more to choose from, and would gravitate towards those that produced entertaining and engaging content. Those that invested in better equipment and taught themselves how to edit their videos more professionally would stand out even more, to the point where you can have
a few drunks lampooning shitty B-movies for over an hour and get hundreds of thousands of views per video on a consistent basis. Hell, you can even do essentially what Bob does in just talking about a movie for ten minutes or so, couch it in a more entertaining persona, and
blow his performance out of the water.
Bob, meanwhile, didn't really capitalize on his initial momentum. Sure, he managed to land jobs at various nerd-centric outlets, but none of those ended up truly lasting, either because the site itself was bought out by new owners that didn't keep him around (ScrewAttack, Geek.com), he got himself fired due to not learning how to shut the fuck up and stop antagonizing his audience (Escapist), or he was simply not worth keeping around any longer (Escapist the second time). His Patreon has been supporting him all this time because
somehow there are over a thousand people dumber than MovieBob with enough disposable income to throw a few bucks a month his way, but he's really lucky that his living situation allows him to use that as his only source of income now that he's been fired from yet another steady gig. He's still making the same videos he always was, with the same production values and the same retarded opinions, and he's barely getting any views out of it, lucky to get into five figures. His subscriber count has been on a downward trend (though we can't see exactly how much thanks to Youtube obfuscating the real numbers), so he's really not building an audience anymore, and with the views he's getting, only about 10-20% of his subscribers are actively watching his videos. Who knows how many of those are decade-old accounts that haven't been signed into in years?
It's safe to say this isn't what Bob had in mind for his life's trajectory. He's mentioned before that he wishes he could have had the same career path that James Gunn did, going from schlock indie filmmaker to mainstream high-profile director, but instead he's stuck in the same rut he's been in for over a decade. He wants to be making movies, not just critiquing them, but he's never managed to move past where he first got started. He makes videos that barely anyone watches, he fails to grow his audience despite being promoted by big and big-ish names multiple times, and he no longer has anyone backing him up.
I dunno if Bob does in fact have narcissistic personality disorder like
@Mola Ram suggested, but the fact he fits basically every description of the disorder sure does lean heavily into that. It also would explain why he's such a major antagonistic asshole to everyone that disagrees with him politically. He believes that his lot in life isn't the fault of his own laziness and refusal to improve, but actually the fault of the bullies of the world. Originally, this would be those who teased him in school, but he's come to associate "bully" with "anyone who isn't onboard with my personal political views 100%." He always had these tendencies, but like many other lolcows, Gamergate would be the impetus for him to start openly stating his hatred of the obsoletes, with Trump's election completely removing his filter. He seeks companionship with those who he believes should be his fellow travelers in the Superior Future™, and he wishes death and destruction on the Mayonnaise Ghouls from Sisterfuck County, Methsylvania who stole
the future he earned. Entitlement seems like it's not even close to strong enough a word for Bob's beliefs.
I think that's what makes Bob fascinating as a lolcow, even if you have to take this thread in small doses sometimes. The difference between how he believes his life should be and how it actually turned out is what fuels his seething bile towards anyone he believes is standing in the way of his success. It's hilarious how he completely fails to understand that his problems are entirely his own doing. Truth is, he'll probably never figure this out on his own, so we'll likely have content for years to come. Assuming, of course, he doesn't die of an anger-induced heart attack if Trump gets reelected.