A lot has been made of the psychological mindset behind police brutality/police riots. See, for example, James Fallows. And, quite deservedly, police assaults are to be condemned. But I'm less inclined to see the question in mainly personal terms. Think back, for example, to Philip Zimbardo's work in The Lucifer Effect, or to the earlier Milgram experiments. Isn't it reasonable, and, indeed, much more fair, to assess police behavior as an expression of the constraints their own system imposes on them? Seen in that light the important question really is: Why does "the system" want to deploy indiscriminate violence against protesters?
To answer my own question I think it's fairly obvious — "the system" is most threatened by non-violent mass action so, to "defuse" the threat, so to speak, the police must instigate counter-violence. If they can manage to do that, because they have a virtual monopoly on force, they will win. But if they fail, the non-violent mass movement has a chance at reaching a point of overwhelming dominance. That, at least, is the theory, and in fact it's been shown to work in many, many cases around the world.
This is why I believe some leadership among the Occupy movement is necessary. To preach and to constantly uphold the central priority of non-violence. Others may argue that a leaderless, unorganized movement can adhere just as well to non-violent action. Color me skeptical.