My personal feeling is that even the majority of 'bad' cops start out with good intentions. I can't help but think of the scene in Crash when Matt Dillon says to his idealistic rookie partner "Wait until you've been on the job for twenty years".
Being a cop is such a thankless job, with low pay, and high risk. Combine that with the fact that they have to deal with the dregs of humanity on a regular basis. That's not a defense of those who become corrupted. But I have the utmost respect for the ones that don't.