[jfengel]

While I hate to sound like a conspiracy theorist, there have been demonstrated instances where that behavior comes from trolls trying to stir up trouble and cast doubts on the merits of social movements. BLM even has a web site where you can report it:

https://blacklivesmatter.com/help-us-fight-disinformation/

I have no idea if that's the case here. I've also known over-zealous supporters of movements to do the same thing.

I'd just caution you to be skeptical about apparent misbehavior. Our existing animosities are being amplified, sometimes for lulz and sometimes for political gain. This person was, undoubtedly, a jerk, but exactly what kind of jerk may not be quite so clear as it might seem.