To a point, I believe police do abuse their power. However, for every bad cop, theres 1000 good ones, you just never hear about them, because they're doing their job.
However, a lot of those videos, the cops are in the right of way. It is a cops right to use force if they feel threatened. I highly doubt in all of the recent cases they felt threatened, especially the one about that high school girl that refused to leave the class. BUT in cases like the Ferguson one, the man had already tried to steal the cops gun, and autopsy results, including the private autopsy ordered by the mans family came back saying the man must have been lunging at the officer for the bullets to go in the way they did....
Take a moment now and imagine...
You are alone in a car, you get a report that a store has just been stolen from, a man matching the description is walking toward you. After a few words, the man attempts to steal your gun. The gun goes off. You're not even sure if you've been hit. You might be about to die... Then you realize the man's hand was shot, not you. He starts to run away, you get out of the car. Your body is shaking, your mind is racing, you're in a state of shock. You yell at the man to freeze, he turns around and starts running towards you.
But that's enough about Ferguson. It's clear the cop was afraid for his life and was within the law to shoot; if he was shooting to stun or shooting to kill was his choice due to the amount of danger he must've believed he was in. (it is advised to shoot to stun, especially if the person is unarmed, but it is NOT the law to do so)
Let's take a look now, at the black man who was killed for selling single cigarettes outside of a store. I do not know much about this case, so correct me if I am wrong...
From my understanding, though I do not know if it is fact, he resisted arrest BUT HE DID NOT MAKE HIMSELF A THREAT. As far as I know, he didn't physically touch an officer, expect perhaps attempting to free himself. THIS was a case of police brutality where the officer was in no danger but chose to use excessive force regardless. Now some would argue that resisting arresting is making yourself a threat... I don't really have a stand point on that; I usually look at the evidence and just pick threat or no threat. This man was very large and his size could have been scary for the officer and made him feel in danger when the man was trying to get away... I do believe I read somewhere that he refused to leave the area where he was loitering and "illegally" selling cigarettes, which I believe (again, this one is all just speculation for me) is enough to get someone detained/arrested.
So, really, it's hard to tell the good from the bad sometimes... There IS brutality problem, and I've read PLENTY of articles about white, unarmed citizens being killed by police. The thing is, it just doesn't get to the news. My personal belief on that is that they don't want us to know. If it were public knowledge that all races are being treated equally badly, they wouldn't have an excuse to fall back on. They wouldn't be able to make things about things that they are simply not about - Though I do not fully understand what those things are or what they're about, it's a simply hypothesis I have on the situation.