In my experience, online "trolls" do what they do because it gives them a sense of empowerment that they don't have offline.
Oftentimes, they are themselves victims of abuse for whom trolling offers an emotional outlet. As the saying goes, misery loves company, and happy, well-adjusted people don't tend to be trolls.
What trolls ultimately seek is control over the behavior of others, and they enjoy nothing more than watching people dance to their antics like puppets on strings. The stronger the response, the greater the feeling of control.
Reactions give them relevance and constitute the payoff for their efforts. Hence the inadvisability of rewarding them with histrionics.
Which, of course, is the last thing anyone wants to hear when they're in the middle of a righteous crusade, so they tend to reject such advice and sink ever deeper into a self-feeding spiral of indignation, insularity and denial.
That, in turn, reinforces the power of trolling, and thus the symbiotic circle becomes complete.