Canadian researchers recently found people who troll the Internet are more likely to be sadists. Named after Marquis de Sade, sadists are people who get pleasure from inflicting pain on others.
In a recent paper published in the Official Journal of the International Society for the Study of Individual Differences (ISSID), Erin Buckels, Trapnell, and Paulhus found correlations between "Dark Tetrad" personality traits and trolling. The "Dark Tetrad" comprise of sadism, psychopathy, narcissism, and Machiavellianism.
In particular, the researchers found a stronger relationship between sadism and Internet trolling. In other words, sadists enjoy trolling because it gives them pleasure.
You can read more of the findings HERE and HERE. The actual report can be found HERE.
Sources: E.E. Buckels et al, "Trolls just want to have fun," Personality and Individual Differences, 2014
http://www.motherjones.com/blue-marble/2014/02/internet-trolls-sadists-psychopaths-lulz http://www.theregister.co.uk/2014/02/12/study_shoes_that_online_comment_trolls_are_sadists/ Wikipedia