After having seen the movie, reading the script really puts a spotlight on Joker’s character arc. It is reminiscent of Travis Bickle in Taxi Driver or Jack Torrance in The Shining. All three characters end swallowed up by their shadow. In a weird way, it represents a unity arc as the shadow aspect of their psyche, which they have tried to deal with and handle via prescription medication or in the case of Torrance stop drinking, emerges into their consciousness and overpowers their will.
In Joker, we even see it in the script’s use of name change: For much of the script, he is called Arthur. Eventually, his character name is Joker.