Yeah, Nextdoor is astonishingly vile given the non-zero chance of actually running into one of your fellow members at the grocery store.

My hypothesis is that Nextdoor primarily appeals to those who already have that kind of territorial "they're all out to get us" mindset since territoriality is literally what the app is about. It's the angry "get off my lawn" old man of social networks and it attracts exactly those kinds of people. Mix in some Internet depersonalization effects and you get something pretty nasty.