why did he just decide one day to become a bad director?
his british movies are "movies" with a wink, an asterisk, a mulligan, whatever your preferred term is, and then his american movies were judged on the basis of being "real movies", a standard to which they come up very short his early work also largely reflects his life experience, such as it is (being a wastoid geek manchild). whereas his latter work is largely just riffing on the movies he grew up with and saying, "here's a variation on what i like." which is enervated and insipid and boring.
peaked with hot fuzz, didn’t know what to do. Shaun and Fuzz were both genre parodies, of stuff he clearly knew a lot about. Then he had to make his own actual movies and flopped like a fish.
He's not a bad directorhe just chose a bad source material