Haha, that is naive, or perhaps incomplete. Actually the "worst" trolls simply tell some uncomfortable truth (which isn't false or ignorant, and he who respects truth wouldn't dare call it harmful).
That is to say, sometimes trolls disrupt communities that are formed around "group think", consisting of members who get together just to be around others who think the same way and avoid those who disagree with the shared ideas.
If you surround yourself with others who confirm your opinions, then anyone who busts in with a dose of reality is a "troll".
There are many forms of disruption, with many motivations, that all get lumped into the "troll" category.