There is a lot of stupid shit being said in this thread. No, no one wants to live in a policeless, anarchistic society. But the current system is failing miserably as well.
Basically, it all boils down to accountability. Police have been shown time and time again that they will not be held accountable for their actions, regardless of how egregious they are. They are literally getting away wth murder, which ultimately perpetuates the problem, starting a snowball effect.
And the public has tried to put faith in the proper channels of resistance, trusting in court systems to bring justice, but the courts have failed them on every account. Protesting is shown useless, and often broken up, or demonized by authority figures... Lumping protesters and petitioners together with conspiracy/anarchist hooligans, belittling the righteous cries for justice as "America hating liberals."
So what are people to do? I don't necessarily agree with retaliation killings... But I understand why people are so angry. Some people feel like there isn't anything else to do but fight with violence. The peaceful tactics have made no ground, and the American public is told time and time again that their authority figures are above the law, and that we are just supposed to live with it.
I don't have an answer. The justice system has failed us, over and over. I don't neccisairly think violence is the right answer, but if you have a better solution, there's an entire country waiting to hear it.