Early on, it’s established when he watches the Murray Franklin show with his mom and he fantasizes about being at the show, that he kind of idolizes Murray. Going as so far to imagine a scenario where he gets singled out of the audience, bonds with Murray who even says he’d make a fine son before hugging him live on TV. He clearly views Murray as a father figure, so he feels completely betrayed by Murray when he pokes fun at him on the same show he watched every night, hoping he could be there. Mid movie, when it’s hinted Thomas Wayne could be his dad, he views Wayne warmly and vistfully, hoping that the man who could save Gotham could maybe save him too. Just to get the work all the way around in finding out that not only is Wayne not his father, but that his mom is a nut who made it all up and that he was an orphan, before socking him right in the face. Couple those shocks to the system with the revelation that he was abused by a father figure early on in his life, and it really helps paint the picture as to why someone like him would fly off the edge how he did, especially in addition to everything else we saw on screen. Right after all his father figures let him down and he finds out his mother let him be abused, he immediately kills her, then sets out to kill Murray and indirectly (maybe) gets Thomas Wayne killed too.
how would that work with a new early 30s y/o Batman and Bruce Wayne in Joker was born in the 70s? Also Arthur/Joker would be an old man fighting a prime Batman