Most of them are good though, just as there are good and bad teachers or good and bad teachers.
I was talking with a guy at the gym who had been a cop in New Mexico, and a sheriff's deputy in MA. Good guy. At one point, I asked him what percentage of cops had major authority problems versus what percentage were regular guys doing a difficult service. I was rather surprised by his answer--he said first, "It depends on where you are. Big cities are worse." (No surprise there). Then he said, "Mmmm... about 35%." So, from a guy on the inside, 1 cop in 3 has at least some attitude problem.
One data point only. As a consumer of police services via the occasional stop-sign type traffic ticket, I'd have guessed a little lower.
It seems sometimes people forget we are all human and we are not made perfect simply because of a job title.