I'm pretty sure lots of BLM supporters condemned Xavier Johnson... It's true that some activists tolerate and accept violence within their cause but that is not universally held, and while issues with police violence do complicate things, I agree with you that violent acts deserve condemnation.
The key problem you highlight with my analogy is not quite correct (although most if not all analogies have some flaws). There are various attempts at organisation and leadership in the Church, but there are also thousands of splits, sub-groups and even violence that has not always been universally condemned (such as in Northern Ireland where I'm from), as well as sanctioned in the Crusades.
An organisation that can have condemned violence while having encouraged crusades, and have members like the IRA does certainly seem like the kind of varied decentralised (or at least multi-centred) organisation that can have internally conflicting beliefs yet frequently be considered as a single entity externally. That was more my point. If you think BLM isn't a broad church, my guess is you don't know that many BLM supporters? My experience has been that a wide range of people support BLM, and most of them do not support murderous rampages, even if they can comprehend why the anger that manifested might have existed in the first place.
My experience in Northern Ireland also makes me extremely cautious about the mistake of retaliatory violence, but anyway. I'm still unconvinced that you could call BLM a singular organisations with all actions done by association attributable directly to it.