1. They would have claimed that Rittenhouse was on their side and disavowed any connection with Joseph Rosenbaum, Anthony Huber and Gaige Grosskreutz. Why? Mostly because it would be strategic. The latter were trouble makers who go to ostensibly peaceful protests to engage in violence. They should be trying to prevent troublemakers from joining their protests, but then, if they were really trying to push for positive change, they would want the protests to be peaceful. The fact that they welcome and defend violent protesters makes it a fair question? Do they actually want violence? I think the answer is clearly, yes, they want violence.
2. They would protest black deaths, not just felons killed while resisting arrest.
3. They would enforce peaceable protests, to make sure they stay peaceful.
Based on polls, Democrats overwhelmingly support BLM in spite of the violence which has been common and Republicans overwhelmingly oppose BLM, because of the common violence, looting, arson and the fact that BLM only care about black deaths when they happen to be felons getting killed by resisting arrest. If they really cared, they would be studiously non-partisan. But they don't care. They exist to get black people to vote for Democrats.