Let's turn that around. How many black people do you think need to die at the hands of racist cops before the State does what it should do and puts a stop to it? 1, 10, 20,100?
There is a massive power-imbalance here. The State and the police are invested with enormous coercive power - politically, socially and, indeed militarily - and with power comes an equally great responsibility not to misuse it. It is being badly misused in the US with regards to black people. The responsibility for all this lies squarely with the government, police and State apparatus that has repeatedly failed its black citizens over decades - even centuries, one could argue.
People don't particularly want to go out and protest and riot in my experience. But when pushed far enough by systemic failure of all aspects of the police and justice system, they do. I support the right of citizens to oppose an oppressive, unyielding apparently un-reformable government when necessary.
You know, it's really easy to solve this problem. The people in power just need to do the right thing and live up to their responsibilities to their citizens. If they won't - and it appears 70 years has proven that they won't - then they need to be made to.