You might want to check out what Southern Poverty Law Center has to say about Johnson and black separatist groups:

https://www.splcenter.org/hatewatch/2016/07/08/dallas-sniper-connected-black-separatist-hate-groups-facebook

During negotiations, Johnson told Dallas police "he was upset about 'Black Lives Matter." That doesn’t mean BLM is to blame. I would lay more blame on the media for inflaming the white-cop-shoots-unarmed-black-man narrative.