How does the violence set back our message
To an extent, wanton violence doesn't promote the intended message either... Violence can be a specific message but only if it targets the specific people at fault and everyone else is aware what the target did to beget the violence. For example, if everyone knows you snitched and you get shot by the friends/family of the guy you snitched on, that sends a clear message that's understood even by the snitch's friends/family.
They might not even retaliate. If you kill someone random from the snitch's neighborhood though the message is changed even if the intention is the same and violence that is perceived to be random begets more random violence.
If the target is white people and businesses in general it becomes a message to white people in general. Obviously the majority of them don't see themselves as enablers/benefactors/perpetrators of a white supremacist society so they feel unfairly targeted and will likely feel vengeful/vindicated when another cop kills another black man. If you target the cops some white people will get the message... Even that has limitations though because the city cops throughout the country that are doing the shooting tend to live in suburban white communities where their cop status gives them respect and admiration.
Suburban white people have an ingrained respect for the "protectors of America" (soldiers/officers/etc)... City white folks are typically a different story. They don't share the same reverence of cops because the cops largely don't come from the same neighborhoods as them and the corruption is much more obvious.