Read this:
A Note on Personal Responsibility.
Humans, in an primal hunting pack like feral yappy dogs. Each individual does little, is even weak, but as a mob, they can run anyone down, exhausted. Like water on a stone, each tease, taunt, spit, poke, kick, vandalism, mockery, etc is just one more drop in the same place.
That's what bullies are, safety in numbers, each able to disclaim responsibility for the damage they wreak on people's lives, the scars they leave on people's souls.
They feed on it.