Just got back from seeing it myself and have to agree.
Holy crap, what a movie. My initial reaction when the credits rolled were at how dark and grim it was. Phoenix gives an absolutely sublime performance.
I've seen various reviews online from critics saying it incites violence and whatnot, but frankly if someone is on the verge of committing terrible acts then any one thing can tip them over the edge, this movie won't necessarily be that thing. Apart from what I would consider to be a nitpick of the fact that I think it portrays the wealthy in slightly too much of a cliché, trope way where they are all unlikable (which A. May honestly isn't that far from reality with the characters portrayed and B. I think is clearly done on an exaggerated basis to make Arthur's character more sympathetic, because it is a movie afterall), overall I don't think this movie in any way celebrates the violence.
Whilst they may be essentially one and the same, this movie isn't so much a celebration of the birth of the super villain, the Joker, as it is a tragedy of the downfall of Arthur Fleck. It's a harrowing tale of an absolutely broken man.
I may have to go see it again.