In the case of the black men involved, that would include the police. Do you think it would be ok if the black men treated the police the way you treat your enemies?
Probably a matter of context and lawful authority. I've experienced an army in a foreign country moving into a university and killing educators. In fact, it's has happened many times even in recent history where the dictatorship (the group at the top) has decided intelligent people, and a free press were "illegal," and proceeded to eliminate them one way or another. About all that can reasonably happen at that point is to remove those people from the area, which is difficult, dangerous, and limited. In smaller countries there is not much of a separation between the army and the police. So that's about context.
Talking about lawful, legitimate, or rightful authority is full of philosophical contention, so I'm going to avoid it.
Will I stand with the black (men) population against the police? No. First, it's a false assumption with no merit. The words black population can only be used in some population statistic. The totality of black people are not members of some grand organization united against the Government of the United States. That organization doesn't exist, so the question is moot at best.
Will I stand with pro football players protesting against excessive police violence?
.....Not while the National Anthem is playing. Why not? I think it's unethical.
Would I stand with people (black or otherwise, pro football players or otherwise) against excessive police violence anywhere at any time?
.....That's more likely.
What do you think should happen?
.....I think local law enforcement services should have a more thorough screening process for entrance, and a more thorough training process before active service. But then they must do their jobs, and hopefully that would include community support.
In other words, it's one thing to protest excessive violence by police, and another to protest the existence of the police force itself. The police force is a much needed element of the criminal justice system. Here's a link that might be informative to someone:
https://en.wikipedia.org/wiki/Law_enforcement_in_the_United_States
There are a bunch of problems in this country, and this problem is on a low rung of the ladder. Economic disparity is higher. Corporate totalitarianism is higher. Access to medical care is higher. Environmental wasting is higher. I could easily produce a list of things more important than excessive police violence. What most of it comes down to though is the need for major changes to occur in U.S. and World activity that should tend to caring for life itself through the social systems that are available, or that need to be built or maintained. Resource grabbing, the me me all about me, and everything is about money ideas need to go. Our politics and social systems should first ask, How are you?