Entertaining, but missing a key piece of the Joker's origin story

Rating: 7/10 (6 October 2019)

I give this film 7 stars for the entertainment value it offers to comic book fans: it rivets and delights us with a series of absorbing, well-crafted, competently executed scenes about one of the most revered and iconic villains in the DC Comics universe.
The acting of Phoenix, De Niro and Beetz expertly straddles the line between the sensibilities of a modern movie-going audience in 2019 and the campy, 1940s comic-book villain reality of the Joker's story (presented to us in the gritty atmosphere of New York in the late 70s / early 80s), while the writing of Phillips and Silver easily draws us into the psychological world of their main character.
Where the movie fails, however, to reach from "decent" to "excellent" is within the frame of its own stated purpose (as related in interviews with writer / director Todd Phillips): the film is supposed to render a realistic account of how a relatively unremarkable man like Arthur Fleck might have become the super-villain that we know as "The Joker." Unfortunately, the movie ends with this task only partially complete.
The movie successfully describes the first step of Arthur's transition. It shows Arthur's physical, mental and economic vulnerability in the context of an uncaring, exploitative society; it shows a human being in Arthur who is preyed upon by greater forces in his environment; and it shows Arthur's (relatable) reaction to these forces. It is a violent reaction, one which transforms him into an "early stage" Joker: a physically and mentally scarred individual who has learned to use violence against those who would harm him (subway scene), obstruct him (Wayne manor scene) or seek to profit at his expense (apartment scene, talk show scene).
Where the movie disappoints us, however, is that it never delivers the final piece of the puzzle of Arthur's transition and The Joker's character: what is it that drives Arthur / Joker to use violence for its own sake (e.g. final scene)? What is it that pushes Arthur beyond self-defensive reactions to his environment, into the perversely artistic appreciation of violent chaos and its possibilities that we recognize wherever the Joker appears in our favorite Batman comic books?
In recent interviews, both (director) Phillips and (actor) Phoenix admitted that they did not have a full grasp on Fleck's / Joker's motivations during the making of this film. However, surely it was a key part of Phillips' and (co-author) Scott Silver's professional responsibility as writers to track the psychological development of their main character all the way to its notorious - and eminently recognizable - final state.
It is for this reason that I subtract 3 stars from my rating: this movie presents us with a job that is only half-done in regards to describing the origin of the villain we know as The Joker. It is well worth watching for comic book fans - and all fans of story-telling - nonetheless.