Earlier quoted context omitted.
Your phrasing is misleading. BLM isn't responsible. A veteran sympathetic to the cause went on a shooting spree granted, but I don't know how you get from that to "BLM was responsible"
You cannot on one hand claim that BLM is an organic, grass roots, and completely headless organization while at the same time disclaiming someone as a member of the supposed unorganized organization composed entirely of people who self identify as BLM members.
It's simply impossible to be definitive with incomplete information, reductionist and I can't see the merit in basing a position in the presumption that all actions are of the whole.