Actually, he is not insane. The accident did change his brain in that he can control more appendages than he was meant to control, but that didn't make him insane. He's evil, plain and simple. I know he's had a tough childhood, and I know he's done a good deed here and there, but he's a bad guy. This is explored in some comics and at least one short story novel. He's a mean, arrogant little bastard and the arms just give him a power trip. It could happen to your most ill-tempered college professor that finds a gun in his desk all of a sudden. He's not different, personality-wise, after the accident from beforehand.
However, this may be different in the movie, as it may be in Ultimate Spider-Man.