The very grave definition of a troll being "a one word term defining a highly dysfunctional online user who targets others using cyber harassment, internet defamation, online deception, cyberstalking, disinformation, extortion, duplicity and cyberbullying when the assailant and target are minors" doesn't fit many of the subsequent definitions. In fact, trolling is extremely common and standard human behaviour and shouldn't be written off as problematic on its own. As with pathology, a certain treshold has to be crossed in order for it to qualify as 'bad'.
Harmless trolling does exist. It's an interesting topic though and I'll comb through it in a bit.