Bob has several Superpowered Evil Sides, with each successive one being even stronger and bloodthirsty than the last.
Downplayed: Bob's a capable fighter, but often he only uses attacks that cause minimal pain. When he gets angry, he has no qualms about seriously hurting his opponent in order to bring them down.
Bob has a split personality disorder. His evil side isn't objectively more powerful, but is much more ruthless, pragmatic, ambitious, and opportunistic so he is able to accomplish more in some situations.
Bob only knows Black Magic and thus is only willing to use his skills when he is evil.
Zig Zagged: Bob has several personalities. There's Evil Bob and Bobby, the evil ones, and there's Rob and Robby, the good ones. Then there's Roberta...
Averted: Bob has no evil personality.
Enforced: Bob's actor wants to show more dramatic range.
Lampshaded: "Bob isn't here right now."
Invoked: Bob is superpowered in his own right; he shows his 'evil' side to gain credibility with a villainous group he means to infiltrate.
Exploited: Bob's Treacherous Advisor gives him a superpowered evil side as a Xanatos Gambit: If he uses it, he will turn evil and thus become Emperor Evulz's henchman. If he doesn't use it, he dies and can no longer interfere with Evulz's Evil Plan.
Defied: "Wait, so you're saying that I have a Super Mode, but it's evil? Screw that, I'll take my principles over power any day."
Discussed: I'll do whatever it takes to save the world, even if it means losing myself to Evil."
Bob is avoided by everyone because he could turn into Evil Bob with no warning. Evil Bob is something of a joke among super-villains because he could turn into Bob at the wrong moment. Both of them are depressed as a result.
Bob is terrified of becoming Evil Bob and avoids people as a result, not wanting to hurt them.
Bob is determined to make the best of it and Evil Bob won't let a puny mortal hold him back.