It’s not just cops. Our society strongly associates black men with violent crime, and unfortunately the police and legal systems play into that: black people are more likely than white people to be arrested for crimes even if blacks and whites commit them at similar rates, and they get sentenced more often and more harshly. There’s evidence that judgment against black people begins far earlier than that, too, as they are more likely to be suspended from school, too. Crazy as it sounds, it’s a phenomenon people have started calling the preschool to prison pipeline. (You can read details and cites for several of those points here. And here.) And it goes without saying that cops often have to deal with criminals and find themselves in dangerous or potentially dangerous situations. So the answer is that it doesn’t have a lot to do with police. It doesn’t mean every cop or even most cops are racist and it doesn’t mean all of these things are intentional. In fact part of the problem is that a lot of these biases can’t be conscious. It’s about society in general, and while individual incidents might not mean much, it’s easy to see that there’s a pattern to these kinds of things. So when there are systemic problems with policing, in the broader scheme that reflects the society the police are a part of.