It's a fascinating film because of the many strange choices being made, absolutely. It was those odd choices that made it worth my time, personally.

The first film, in my memory, borders on abhorrent because of - as you say - a quiet insistence that because the Joker is mentally ill, he can kill people and it's less of a problem somehow. If Joker were at the very least to take his frustrations out on those who actually caused the very root of his problems, it'd be a move in the right direction, and even then it'd be a mistake. The Joker can't be a righteous character because his response to the world is one of extreme violence towards people! It doesn't overly matter what that motivation is, he lusts after supposed revenge and lashes out violently towards others.

That is what this film seems to have a much clearer understanding of. I'll be curious to go back and watch the first film with fresh eyes after having seen this - maybe Phillips was intending to make fun of Joker's ideology of violence, but it never seemed that way to me when viewing the first film before. Either way, this film does side with the idea that there is no Joker, that Arthur Fleck did just lash out against a world that he felt had wronged him personally, and it makes for a really surprising, striking idea!