It was really a case of "be careful what you wish for". Cartoon Network liked how dark the Season 2 finale was, and notes came from them basically saying "Keep it up! Feel free to get darker! In fact, get as dark as you want!"

Also the show's creator is an unstable alcoholic so going from "lol xtians zombies and monkey cheese" to gleefully misanthropy is as much him losing his mind as anything else.

Starburns' commentaries on the Community DVDs were always the worst.