I can't believe it but I agree with UP. The fact that police think that they are soldiers and equipped like soldiers.
On the one hand, yes, cops thinking like soldiers is bad, and certainly isn't helping. You can't protect people in a state of war against The Enemy.
On the other, I'm not sure this is the root of the problem. Militarization of police is a localised problem (in the sense that it's something happening in America right now, not everywhere throughout history), but police abusing their power and being unreasonably protected from consequences is almost a human universal.
I think the problem isn't so much militazation as it is militarism.
A big reason societies let cops beat people's brains out is because humans often feel like the best solution to a problem is to violently target it. This isn't always the reason police actually abuse power, but it's a major reason people will let it happen.
