Even that is a broad topic because the protest has become subject to many topics beyond the original point. But, I'll try.
The original subject/topic of the protest, well I don't see how anyone could have an issue with it. Specifics, scope, history and such will come into the discussion of course, but discrimination, be it systemic, overt or tacit by any organization, ESPECIALLY a government organization (police, local, federal, military, education etc) cannot be tolerated. I don't think it can ever be eradicated (in any organization) because you have individuals, and therein lies the problem. But that doesn't mean it's ok or can be tolerated. Especially when it comes to matters of a personal physical well being, let alone life itself. So, by that, yes, I believe it's a just cause that Kap is trying to highlight.
Another big part of this has become what can NFL do, and should they do, if anything.
My opinion is the NFL can prohibit these protests if they wish. These are employees on the job at the workplace, and the employer has a lot of latitude to say what you can and can't do while at work - just as any of our employer's do. The right to free speech (in the US Constitution) is pretty narrow in scope. It protects citizens from being denied the right to dissent or speak as they wish by the government. You still can't yell "fire" or "bomb" in a crowd under "free speech". Broadly speaking an employer can limit free speech, especially in the work place, but that's certainly not unilateral.
So people who say the NFL could make them stop if they wanted to I think are correct, in a legal sense. Saying they SHOULD is different. I think while the NFL has that right, it would be absolutely the stupidest thing possible, regardless of pressure from stakeholders, advertisers, POTUS, whatever.
It's hard to just give my thoughts on all of this, because there are so many discussions that can dove-tail from this, endlessly. But hopefully those 2 narrow-scope thoughts give you a couple of my opinions?