Recent violent attacks against police officers will likely hurt the Black Lives movement. Much of the public is now very sympathetic to police officers. Furthermore, news about these violent attacks on law officers means there will be less coverage for the Black Lives Matter protestors; and less coverage about issues facing the black community.
Black Lives Matter is supposed to be an anti-violence movement against police violence. Using violence against police gives them even more reason to use violence against any race. Violence against police puts police on edge and makes them more likely to go straight for their gun, before trying another option against a criminal. It doesn't matter what color a person is.
Black Lives Matter movement is a movement that was started to bring attention to the senseless murder of innocent black people all over the country. Its premise is against violence and to see someone take up arms against the police is nonsensical. The gunman was deranged but that doesn't mean there aren't other people looking for a reason to do the same thing. They need to stop it immediately.
While it seems like a huge blow to the Black Lives Matter movement, true supporters will remain loyal supporters. The actions of one murderer or a few violent individuals does not change the facts, the problems faced or the message of the movement. Anyone who sees this as negative towards the entire movement was likely not supportive in the first place.