That is a hasty generalization, to say the least. A good few years ago, the PBS station if SF Bay Area (KQED) run a documentary on police brutality. On of the most striking revelations was that virtually all cases of brutality could be tracked to a handful of individuals - a tiny fraction of the entire force. The scandalous thing about that was that these few thugs on the force were covered up and protected by assorted right-wingers in high places.
If I were to make a generalization of my own - I would imagine that motivation to be a cop is grounded in a desire to serve a public interest, especially in poor neighborhoods. While the police protection may be grossly inadequate and fraught with abuses there, it often IS a thin blue line that offers the only protection for these neighborhoods from disintegrating into a total chaos (which, btw, is NOT the case of more affluent neighborhoods).
As far as military service is concerned - I find it hard to believe that anyone in his right mind can see any bona fide public good being served by this institution. Imperial interests - yes, but public good? I used to work for the US military as an instructor, and my impression was that that most recruits were, in fact, soldiers of fortune lite. They joined the service for economic benefits (mainly college tuition) but they did not expect to do much in return - certainly not to risk their own lives for the empire. I think of them as a mercenary army, a Foreign Legion of a sort. Some of them joined to escape their personal or social problems, others (perhaps a minority) - to find a legitimate outlet for their criminal impulses.
That is, btw, why I think that the "support the troops, oppose the war" approach is utter oxymoron, an idiotic expression of faux populism by the way-in-the-left-field-liberals. Why should anyone support soldiers of fortune? Unlike the conscript army, they joined the service voluntarily for a personal gain, even though they knew, or should have known, that they will be used as the cannon fodder for the empire? Why would that deserve any support?
Wojtek