Oh, I’ve read lots of those comments. Truly fascinating stuff, and I think there is a reason. Here are some guesses.

1. That user is suffering from some sort of mental issues. This seems like the obvious option, but I don’t think it’s the most likely one.
2. It’s a social experiment. Someone is using it to gather information on how people respond to hostility online. I would love to carry out an experiment like that, but can’t justify the negativity that would be inherently involved in it. I’m really curious to find out how different instances, communities or even other social medias respond. Are some places more hostile or mature than others? What does it take to get your messages deleted or account banned? Who knows.
3. It’s a piece of online art. That account is used for making a statement about online debates, hostility and the ways people react.
4. It’s a personal goal. Who has the most downvoted account on Lemmy? That’s an achievement of sorts, I guess. This is the kind of stuff people do when they’ve already completed the main quest and most of the side quests.
5. Plain old trolling. Sort of like point 4, but for entertainment purposes instead of questionable bragging rights.