I think Mysterio is more then thief though, if he was a thief there are way easier and cheaper ways to commit robberies then over the top special effects, I think he commits crimes for attention and fame.
Now when people say they want Mysterio to be unsympathetic, do they want him to be just a jerk or a monster? I agree that Mysterio shouldn't be sympathetic and doing that makes stand out from many of the previous villains, but if he is going to unsympathetic, then he should do something really bad so I have a good reason to hate him, I get invested in the character in how contemptible he is and how much I want to see him get taken down, rather then Mysterio just stealing some marked and insured money from a bank.
I mean Venom from Spider-Man 3 was just a jerk and he was really a lame character. He wasn't sympathetic, but he really wasn't a really evil villain either, he was just some jerk.
Again I think Mysterio as a sadistic mind breaker makes for more of a compelling story then generic thief, because it gives Peter Parker a story arc, having him over come the sadistic mind games Mysterio is playing and becoming a better person for it. If a Mysterio is just a generic thief, there is no real story arc there.