The danger of a forum bully is two-fold. In one respect, the forum bully makes the other respected members of the forum appear hypocritical to an outsider. When the forum bully spouts her own brand of nonsense, when the forum bully tears apart other forum members for no reason other than because she can, it makes other members of the forum look bad - simply because the very behavior a troll is ostracized for, is the exact same behavior permitted to the forum bully. The double standard so created reeks of hypocrisy and undermines the legitimacy of everyone who does not call the forum bully out for her repugnant behavior.
The forum bully is not stupid. The bully will never attack other respected members of the forum. She will also mix her attacks between those who genuinely deserve being attacked, and those she attacks for fun. After all, the only way a forum bully stays in business is with the consent of the other members of the forum. And she knows the fine line between being labeled a troll and being called blunt or to the point.
The other danger of a forum bully is that she creates an atmosphere of fear within the forum - not fear among the established members, but rather fear among those outsiders who want to participate in the forum. It is this fear that enables the forum bully to operate. A forum operates on the classic in-group/out-group dynamic. The bully's behavior is tolerated because of an understanding of this dynamic. You don't attack those within the inner circle, only those outside it.
If one of the respected members of the forum were to call the bully what she is, a petty, repugnant fraction of a human being, this member risks losing position within the forum. The resulting forum is a place in which comments are guarded. Politics are played. And the free flow of information is stifled. This is the true danger of allowing a forum bully to continue her activities. By not calling her out, the forum loses part of the dynamic it was created to foster. The forum bully also undermines the legitimacy of everyone else in the forum.
Online Forum Etiquette: How to Deal with a Forum Bully