Asking people who are treating other groups badly to stop is necessary, whether or not it's divisive.
Telling all of a group that they're treating other groups badly is, I argue, divisive, in two ways. First, it defines the accused group as being a unity which is against the other group, which is also defined as a unity. That's dividing people into those groups.
Second, it's peeling off allies from the other group. People react by thinking "If you're going to lump me in with the oppressors, without knowing a thing about me, then screw you. I'm out. I don't want anything to do with your crusade."
Telling all of a group that they're treating other groups badly is, I argue, divisive, in two ways. First, it defines the accused group as being a unity which is against the other group, which is also defined as a unity. That's dividing people into those groups.
Second, it's peeling off allies from the other group. People react by thinking "If you're going to lump me in with the oppressors, without knowing a thing about me, then screw you. I'm out. I don't want anything to do with your crusade."
I agree with your second paragraph.