There are different types of troll. You find that out on a site which has great personal investment, but no moderation.
This site currently has no real trolls, because the moderators just milk the trolls to create content, and kill them off, when they get tiresome.
When you are on a site that people live on, the whole day, as a social medium, and there is no moderation, an infuriating troll ecology builds up. The most head-banging ones obviously have mental health problems. They just keep banging their tragic deluded cripe out. This causes various members to commit hari kari, and vow to never come back, unless so-and-so is removed, somehow. Then there are the ones who are idealistic social crusaders, and you can sort of see their POV, and you would ordinarily agree with them, but they just unconsciously like rubbing everyone up the wrong way by taking the unnecessary moral high ground on every issue, including their own victimisation. The next type is the oblivious attention whore, who thrives on abuse, and enjoys sock puppeting, and sending psycho threats to the site owner.
That covers the main types of psycho troll. What they have in common, is they absolutely will not go away (in a non-moderated environment)
The 4th type of troll is the deliberate stirrer, and starter of flame wars. They are much less common. They have self awareness.