Mostly it’s a reflection of their own self hatred. There’s a heightened effect on the internet (especially forums) because of the anonymity. You’ll likely never meet your tormentor, and they know it.
For guys who are new to forums and other internet sites that give users protection under the veil of anonymity, it’s hard to realize that most people are just busting balls because you lose a lot of the social cues through text. You’ve got to read between the lines and just know that you’re going to come across some dummies here and there.
The OT on this site used to be ruthless back in the day, and yes people advocated for others to kill themselves regularly. Mods stayed on top of it for the most part. I even recall a whole year where “Go drink bleach,” was a common thing to say (probably 2005-2007) on here and all over the internet. I think it got so bad the the saying, and sayings like it, were written into the rules of the forum as an almost immediate ban.
The new cool thing is to call people a racist, homophobe, xenophobe, bigot, etc if you disagree with them.