There have been countless cases of police brutality around the world, and specifically in the US. It isn't the norm, no, but is most definitely a problem. The ones who have the most power need to use it responsibly. Sometimes a person who has power may have mental issues, not necessarily mental disorders, but problems at home, and problems at work, and you give that person a gun and a badge and a perp to exercise that power on...nasty things can happen. And it's a problem the police force must work out.
While the "all cops are dirty" sentiment is erroneous, it's a problem that exists and police brutality needs to be addressed. The vigilante justice currently going on in California is not the right way to do it, it doesn't help the problem and just adds more violence. Police departments need to take this seriously and punish those guilty of it harshly and immediately upon discovery.
Police brutality is a huge problem in this country. I have witnessed this, even in my tiny corner of the world, more than once, and even though I have never had any direct contact with the law in a criminal way. Police do not receive enough training before they get their badges, and I hate to say it, but it seems that a lot of them go into the occupation with an ax to grind.
Police brutality only seems like a problem because it is so easy to film and share videos now. It is very rare for a video of a police encounter to "go viral" unless one of the parties is misbehaving. Also out of the vast majority of "police brutality" videos I've seen only a handful are actually worth getting angry over. If people uploaded videos of regular everyday police encounters I'm sure people would see it isn't as big of a problem as it seems.
That being said, I do believe officers that are shown to be a problem should receive stricter discipline than what is essentially a slap on the wrist.