Internet trolls are people who find the need to leave comments on different forms of Internet communication to disturb the flow of conversation or create a bunch of ruckus. They're known to exist on all kinds of services, and science has finally taken a close look at these individuals to profile their behavior.
In a study involving about 1200 test subjects, approximately 5.6% of these individuals say they have fun trolling people online. It was found that these people typically showed signs of psychopathy, narcissism, or even sadism, which all explain why Internet trolls do what they do.
What this means is that those who identify as trolls typically enjoy watching the suffering of others. Putting bashful comments on websites that make others upset feeds these urges, and is why Internet trolls do what they do.
Does this make Internet trolls bad people? Of course not. They just get pleasure from a little disruption the same way you might enjoy watching two people fight or wrestle on television.