If my brother kills someone then yeah, that's his fault. Not mine. I can choose to look after the victim's family, but that's a personal choice.
If my country goes into South America and floods an entire country with guns and 20 years later the entire country is a narco state, then yeah, as an American it's my responsibility to not vote for someone who would continue those policies. Or to help the people of that country and not pretend that America didn't play a part in creating the violence in that country just because I was six at the time.
This isn't a issue of blame, it's about acknowledging that the way things are now is based on past actions. "Rome wasn't built in a day". If you don't like the way things are now, then simply acting like the past doesn't matter isn't how you fix systematic injustice like Ferguson or NY.
Cops make mistakes. They are human. Individually, none of these cases mean anything more than mistakes, but put them together and you see that something is fundamentally wrong. A justice system that allows cops shooting someone in a store selling guns for holding a gun is insane.