Arthur ends up back in Arkham, the humanity in him winning out over "The Joker". Harley having abandoned him upon realising he's not Joker, he's Arthur. Except that's not a figure of speech like you may think, neither are the other hints leading up to another inmate walking up to Arthur (one we have seen in the background throughout the movie though his face has been largely obscured until now).

The inmate tells Arthur a joke, before Arthur can answer with the punchline the inmate stabs him to death. The inmate then gives himself the Glasgow smile, revealing he is the true Joker.

A brief director Q&A after the film further implied the events of the first movie may be not what they seem (to us). That Arthur was imagining himself/confusing himself as "The Joker" but many of the actual actions of "Joker" in the first film was this Joker that stabbed Arthur, "the true" Joker. And that one of the few instances of Arthur actually being Joker was the ending of the first movie, where he was wrongfully blamed for the actions of this Joker due to claiming responsibility that led to the trial in this one (and Harley wrongly falling for Arthur).

Also during the Q&A they teased this was a younger version of Heath Ledger's Joker and this was somehow an origin movie for The Dark Knight's Joker