I don't know why people are calling it dark and disturbing. The movie is not so dark and nothing is that disturbing.
The movie follows a comedian Arthur who is suffering from mental trauma and with constant disregards and abuse from society he reached his threshold and becomes The Joker.
The movie gives a origin story of the villain so you won't see any lunatic and chaotic stunt from him.
Overall it's a great flick and I definately recommend to watch it