I'll leave the analysis of the Alton Sterling video and historical context to others. I'm a political scientist, so here's some political science.
Perhaps the most important political scientist of the 20th century was Robert Dahl, best known for developing the theory of "pluralism." Broadly speaking, pluralism is a model of power in which power is held, not by some "power elite," as in C Wright Mills, but by a "political stratum," which is both permeable and shifting. As different issues activate different segments of the population, the resources that are important can change, and the people who can get their way aren't always who you would expect. Dahl wrote a lot, and while he is most famous for Who Governs?, his best book is probably A Preface to Democratic Theory. You can pick it up pretty cheaply on Amazon or eBay. You can't be an edumacated [sic] person until you've read it. Go do that.
My favorite example of power being weird in American politics is the persistence of ethanol subsidies. Nobody likes them. They raise the price of fuel overall, raise the price of food, increase total fuel consumption... You know who does like them? People who grow corn. You know where they live? Iowa. Fucking Iowa. You know what Iowa has? The first in the nation presidential contest. So, most presidential candidates pander to them by supporting ethanol subsidies. You know what we have 100 of? Presidential wannabes. We call them "Senators." Any elimination of ethanol subsidies has to get through those assholes.
That's a weird resource. Power is weird. And when you strip away the bullshit that critics of pluralism try to concoct, that's really what the theory is all about.
So now let's talk briefly about the kinds of power that groups can have in more normal circumstances. There's numbers, obviously. Money is a resource. It is vastly over-rated, but I'd rather have it than not. Organization. Connections. Education. These are the main tangible resources. After that, it gets more amorphous, but I'll introduce public sympathy.
This is where the "Black Lives Matter" movement has a strategic problem when it comes to the difference between cases like Tamir Rice and Alton Sterling. Alton Sterling had a criminal record. Tamir Rice was a kid. Regardless of any other facts of the case, that gives a movement built around Tamir Rice a resource that will be lacked by a movement built around Alton Sterling: a central figure without a criminal record.
In neither a legal nor a moral sense does Alton Sterling's criminal record have anything to do with what happened to him. But in the public's mind, it will.
Tamir Rice. Kid. More sympathetic figure. A strategic movement would focus on that as a public face.
A movement without numbers, money, etc., needs to be built around the most sympathetic figures possible. Kids like Tamir Rice will always be more sympathetic to the broader public than convicted criminals like Alton Sterling, even if the latter was a case of cold-blooded murder. A criminal record wouldn't justify police murder. But the lack of one makes it a lot easier to get the American public on your side.
Your dose of realism for the day.