This was supposed to be the origin of the Joker we know. The director said he doesn't even know if Arthur Fleck is the Joker we're familiar with, or someone that came before him. He said he doesn't know who Arthur becomes, so he's writing an origin with no destination in mind, when he should know exactly who he becomes. That's why he messed up the character the way he did. He thought he had more freedom than he did when developing who the Joker is.
You wouldn't write a batman origin movie that just had him killing people on some Punisher shit because that's not who batman is. But apparently if someone did that yall would just say "Well of course the batman down the road wouldn't do that, but this is before that batman. Something in the future probably made him make that drastic change, and it's ok we're never told what that is. Great movie"