bad guys?

I see your point, and it is very cut and dried simplistic thinking. However, in their job they don't have time to sit around philosophizing and analyzing the shades of grey in between black and white. The law, and their interpretation and enforcement of it, doesn't really allow that kind of analytical and selective processing. To them, and for them, it really is that simple: arrest the person who is violating such and such section, subparagraph, or line of the Criminal Code. I'm sure many cops privately agree with what you are saying, to a certain extent. Also, the psychological entrance exam you mention in Germany is good, because I think it discourages these extremist, renegade types who want to lay an armageddon on the streets. But that won't stop these very same cops, a year, two years, five years, twenty years down the road from developing that thinking because they are socialized and immersed in it by their peers. Policing is very much a "brotherhood" (including female cops) and the culture, cohesion, and bonding is a very important aspect of getting by, surviving, and fitting in on the job. So whatever idealistic or preconceived notions one has going into it, the "thin blue line", plus the convenience in a stressful job of grouping the people you deal with into "good" and "bad" and not making it more complex than that, is probably going to seep in eventually.

SIG:http://www.youtube.com/watch?v=3TacmWwMRaA

The psychology of police training and their internal culture revolves on a few core tenets. No one is a law-abiding citizen, everyone is always lying to you and you are never wrong so we'll support you no matter what. You can see the "wonderful" results of this kind of mindset in police forces around the country.