Did you actually attend a Black Lives Matter rally, or did you get this information from the news media, which tends to present the most extreme and attention getting elements of every group.
The BLM rally I attended was peaceful, charitable and constructive.  The speakers talked about how all lives matter, how black families need to be strong, and how we need to respect the police.  I have been thinking lately about how negative stereotypes develop because we get our information from the news media, not from actually engaging with one another in our own communities.
Don't forget the civil rights movement in the 1960's was not always as nice and pretty as it is now portrayed.  Dr. King was clear that people will change not because they are so nice, but because their way of life and economic privileges become disrupted and this was the intent of the movement.  Dignity and justice have always involved some degree of disruption, hopefully the peaceful kind.