There is a precedent for a victim’s family suing a racist organization of which perps were members.
http://articles.latimes.com/1988-09-20/news/mn-2179_1_beulah-mae-donald
There is one problem with the Donald case. The two men who were convicted of killing her son were actual MEMBERS of the KKK.
BlackLivesMatter is more of an amorphous group without much, if any, of a formal, card-carrying, dues paying membership.
Cop-killer Johnson Yabadabadoo may have attened BLM protests, but unless you can demonstratively prove that he was an officer or “official member” of the group, a lawsuit against them will fail.
A better approach might be for a victim of a BLM created riot to sue them under a RICO charge, or state/local equivalent of.
Also ask the FBI/police to charge them with crossing state lines in order to foment violent because it is both a federal and state charge (under slightly difference state laws).
Jonson’s Facebook postings seem more like a diversion from his real feelings. Or the bastard just self-flipped and went stark raving mad. However, dead men don’t talk.