I agree, and the place to start is training. With the backlash against police intimidation and brutality we've been seeing for a couple of years now in the U.S., some departments are emphasizing an approach of "Guardians Not Warriors" in their police training, both initial and ongoing training.
I hope it continues, and I hope it works. We need to replace the "warrior" culture that sadly and outrageously has been pushed for the last 30-40 years. I think it will help a lot if police are trained as I've outlined. After that, I think regular counseling, both by professionals and by the sergeants, is necessary to identify the bad boys and process them out, and to help the good guys avoid slipping into depression and despair on the way to becoming bad cops.
EagleSmack is absolutely right in one way. Simply adding a counseling requirement will not help. We need a change in the culture of police. They need the kind of pride the Marines have. It should be a matter of pride for a cop to say, as Kevin Vickers did, "In my 30 years I never fired my weapon outside of the training range." Cops should be lauded and rewarded for being smart, not tough.
As far as counseling goes, I think you're right about witnessing violence, but what's even more important is that cops spend a lot of their time dealing with the poor, the ill-educated, the hopeless, the disaffected, and the mentally ill. That's what I mean when I say "avoiding slipping into depression and despair." Counseling, both traditional and force-based (by which I mean "cop-to-cop" talks with the training officers and sergeants) can help cops avoid slipping into a "people are scum" attitude. THAT is the birthplace of police brutality.