Snider clearly had issues
I know that's rather stating the obvious but the guy did clearly has his fair share of demons so to speak, and the film clearly makes some attempt to hint at this. The one moment I recall which stands out for me was when when Snider and Dorothy are at some party or whatever it was they were at and he spots "the jock". It's then I believe for the first time we see how much of a chip in the shoulder he has against certain people. Even though the guy never even so much as look in Sniders direction he just seems to take umbridge at the guys mere presence. This of course leads to his immature act of spite when he stabs him in the backside with what might have been a small penknife.
It strikes me that the guy had been the High School nerd who was one of the outcasts so to speak. He was likely taunted, bullied and doubtless humiliated by the more athletic, good looking male pupils and looked down upon and sneered at by girls who wouldn't even give him the time of day. Although the scene where the loan-shark (or whatever he was) had two of his goons dangled him out of that apartment window while several storeys up could have brought about flashbacks to his high school days.
Anyway, the way I suspect girls g=regarded him in the past is largely why he becomes enamoured with Dorothy and wants so much for her to be successful. Her being so would somehow logically make him someone of worth seeing as she chose to be with him. Ridiculous, I know because she had decided to be with him before fame and success began to come her way. However rationality never really seemed to factor in to Snider emotionally or psychologically. Ultimately, he's more of a pathetic, pitiful character in my eyes than he is hateful. Easy for me to say, especially since he ends up blows her brains out with a shotgun before cowardly turning it on himself. He's definitely the movies "villain" of the piece but something of a tragic one from my perspective. Dorothy on the other hand is the tragic victim although one might argue she should have had more sense than to meet the guy on her own. Not that I'm for one second excusing what he did but never the less.