If he didn't post on instagram, it would be non-existent.
This seems like dude shot his girlfriend and afterwards felt like he had nothing to lose. He was going to kill himself and was determined to take others with him.

The protestors should not feel that they have to explain his actions. Explaining it as a direct result of the non-indictments is probably the worst position anyone could take.

I think the best response is that he is just a terrible man who did terrible things and express sympathies for the victims.