If you're talking about social media, anyone can post something there claiming they're part of BLM. It doesn't mean they are. In the beginning, BLM was loosely organized. In our area (dealing with the death of Tamir Rice and others), they were approached by local community organizers who were experienced in activism and civil disobedience, offering them advice and guidance. Early BLM leaders disagreed at first. They weren't wanting to engage in violence against anyone, but some were willing to break some windows and cause havoc. Over time, they developed good leaders and adopted good guidelines for peaceful, organized protest. They did so without losing their passion for their cause.
Since then they've learned a lot, including learning how to watch their protests for troublemakers, distance themselves and alert LE. Sometimes these troublemakers are anarchy types, other times they're plants or agents provocateurs. Other times, they're violent counter-protestors. There are usually people in every protest march whose job it is to watch for troublemakers. They also hold regular meetings and have learned to spot bad actors and remove them. There's real skill in doing this.
In Dallas, it looks like the killer avoided interaction with the protest and BLM. IMO, it looks like he positioned himself away from the parade, where no one noticed what he was doing. I'd also be willing to bet he never attended meetings or other activities.
