Taboos around violence for political are one of the crucial building blocks for a functioning democracy. If those taboos are broken, even for a good cause, you set a precedence that violence works. And the next cause won’t be as good. One only has to look at the lessons of the Roman Revolution that started with the murder of Grachus, and ended with an Emperor who everyone acclaimed as they were so tired of the bloodshed.
The non-violent protests of Colin Kaepernick were mocked and used to rally the other side and just weren't effective.
The problem here is not the violence, but a policing system that is so fundamentally damaged and has not been effectively reformed fast enough.
The MLK quote is trotted out pretty often, but "a riot is the language of the unheard".
Yesterday police in Columbus, Ohio replaced the American flag flying outside of their headquarters with a "Blue line" police flag. Just hours ago representatives of our nation's largest police force, the NYPD, posted on twitter the home address of the mayor's adult daughter.
It's a lack of political will, but that's kind of putting it mildly- it would be more accurate to say that the politicians are afraid of the police.