Make no mistake: Neither the fictional character Joker, nor the film, is an endorsement of real-world violence of any kind.
(The film did play at the Venice Film Festival, where it won the coveted Golden Lion, and at the Toronto Film Festival, where it garnered largely positive reviews.)
At the same time, Warner Bros. believes that one of the functions of storytelling is to provoke difficult conversations around complex issues.
It is not the intention of the film, the filmmakers, or the studio to hold this character up as a hero.