A recent study on internet behavior found most people who engage in what we would normally consider to be "trolling" tend to also exhibit the personality traits similar to psychopaths.

Referred to psychologists as "The Dark Tetrad", these traits include things like narcissism, Machiavellianism, psychopathy and a sadistic personality. Slate reports:
The study found correlations, sometimes quite significant, between these traits and trolling behavior. What’s more, it also found a relationship between all Dark Tetrad traits (except for narcissism) and the overall time that an individual spent, per day, commenting on the Internet.
It all makes perfect sense now, especially once you realize that normal people wouldn't be spending all their time to actually "troll" anyone on the internet.