Internet troll.

An internet troll is a person who intentionally upsets others in social media forums. The troll’s actions, called trolling, are generally understood to be undertaken to cause consternation. Many post inflammatory or malicious comments on social media as a means of gaining attention and generating comments. The use of the word trolling is believed to refer to the action of fishing by trailing a baited line behind a boat. References to online trolls date to about 1992, and the origin of the reference, whether to the act of fishing or the monsters of folklore, remained uncertain.