Are they just shitty people? It seems weird to me that people just love being mean. I know a lot of kids were like that, but I figured that people grow up and get to a place where they just run their own lives.

It's because the people they're mean to have been sorted into outgroups that they don't really consider human. Many of them are perfectly kind to anyone in their ingroup.

they're addicted to outrage and desperate to feel better than some other class of person.

Yes. The shittiest of people.

Yes, they're just shitty people