I suspect the answer is that there’s probably a depressing number of authentic human adults who just are like that, and it creates a feedback loop/spiral where people are pushed into being more aggressive/vitriolic as a defense mechanism.

The real problem, I think, is the ease with which those individuals can hop between communities/be directed toward communities particularly sensitive to their brand of bile on social media sites. I know there’s a lot of talk out there about making on-boarding to Fediverse stuff easier, but realistically, being able to layer several barriers along the way (e.g. finding an instance to join, finding an instance to harass without getting either yourself banned or your entire instance defederated) will go a long way toward limiting the influx of bad actors.