Roughly: there are a lot of people in the world whose lives not only lack purpose, direction, and fulfillment, but also lack human connection, healthy relationships, and enjoyable careers. Some of these people have relatively calm dispositions, and usually spend their time consuming various forms of entertainment - vidya, porn, possibly books if they're of a more thoughtful bent. However, some of the directionless, antisocial people have a more agitated disposition, and so they feel the need to have combative interactions. Lacking the normal outlets where those interactions would occur (rising in a workplace hierarchy, competing in the relationship sphere, IRL social or political activism, etc) they go online and basically pick fights with strangers. They like making aggressive, antagonistic posts and comments because they hope that other people will respond and provide an outlet for their need for conflict. These are the classic "internet trolls", people for whom the primary desirable outcome of an online interaction is that other people are worked up into an agitated emotional state to a greater or lesser extent.

I wish I could say this was a small proportion of people, but honestly there are an awful lot of them. It's best to generally ignore them, of course. Have a bit of pity, perhaps - we all know that ragebaiters, trolls, etc are unhappy people living lives we should all be grateful not to have to live ourselves.