Internet Cruelty

I have no context in my life experience that helps me understand people who go out of their way to write cruel things on the Internet about other people. I can't see the benefit in any way - personally or societally. I understand being in a bad mood and ranting or being angry and blowing things out of proportion. I don't understand writing that a person is ugly (et al) as proof that what they say, think, or do, is not worthy of respect. There's an abundance of evidence for it so I know people are like that, but why? What does it gain for them?