There are some people out there on the web whose sad, empty lives revolve around harassing others online, delighting in the misery they cause by mocking, criticising or belittling others and their opinions.
We call these people – among other words that are inappropriate in polite company – ‘trolls’.
By and large, trolls are little more than a nuisance; you get used to seeing the same usernames popping up, and you learn to gloss over them in the knowledge that they have nothing useful to say.
But there is a particularly evil breed of troll, who persists in targeting vitriolic abuse towards someone, to the point that their life becomes miserable.