Bards aren't the only class that like that tho, any class can either help or hinder, gamzee was just insane and fucked shit up due to lil cal, not his class.(totally different anon btw)

But his class was involved.  He fueled rage into Terezi which lead to her killing Vriska.  He made Terezi hate him which lead to their kismesitude. In his berserk mode he let the rage of other be diminished by terrifying them. You can read the class page on the wiki for more detail but his class was definitely involved. And I know that other people have written longer explanations so you can look for those if you like.