I think it's part of his character.
He's like Pee-wee Herman, or Larry the Cable Guy, he has that character, and that character is supposed to be rather cynical, sarcastic, and at time unpleasant (a kind of humor I can appreciate); I've heard stories from people who've met him IRL, and say he's just as big an *** in person as he is on the internet, but again, I think that's just part of him being in character.
I've heard rumors that when he's just being himself (as in Doug Walker, and not the Critic, or the Bum, or anybody), he's a really nice guy, but there's really no substantial recounts to back that up, as, again, most of the stories I've heard of people meeting are pretty much him being in character as Nostalgia Critic, and acting like an *** with his fans like he does in his videos.