I believe the main "gotcha" is that black on black violence, especially gang violence, is not a main focus of Black Lives Matter. It is a whataboutism meant to distract from the point.
Gang violence is a large social problem, and one that the black community does need to deal with.
Flipside, one of the reasons for the issue to have gotten as bad as it is, is the lack of trust in the police force the black community in general has towards most law enforcement, and as highlighted by the Black Lives Matter protests, with good reason.
Until people can go "I can call the cops on this issue, I'm not going to be shot or assaulted for reporting a crime." The criminals in their community will have greater leeway to do as they please. And that includes killing each other.