A group of people that together, in numbers (a safe position), has the personality of a bully.
The collective bully was waving at and teasing that guy.
A group of people that thinks they are nice and unselfish, but is really a bully together.
He had broken free of the chokehold on his mind (and body) of the collective bully.
A group of people that tries to push one person around.
A collective bully is still a bully, just like a collective asshole that seems like it has many nice and unselfish parts is still an asshole.