The difference between a cop and a perp is opportunity.
Any cop worth a damn is going to be shady. To catch criminals, you have to be able to think like one. All those years of busting dope boys and hustlers...you see all that money, those cars etc. Cop (especially a white one) thinks he's entitled to the good life.
Big advantage is that he knows exactly how the police operate so it's much easier to stay under the radar.