Less than 1%. Despite what you see in Hollywood movies and left wing blogs, cops rarely commit major crimes. The worst crime most cops commit is driving 10 mph over the limit on their way home, which is something every damn person on this forum does also.
You'll find the bad apples and outright criminals wearing a badge here and there, sure. Any group of humans will have bad ones. But NOWHERE near the number that you, and guys like Obama's friend Bill Ayers, believes. You all just like to pretend thats how it is because basically you hate cops, hate them because they make you do shit you don't like to do, or won't let you do shit you want to do.
Or as my former Sgt at Atlanta PD said: "If all the citizens could 'speed and smoke weed' we'd never have a complaint".