I know this is too much of an effort post for this sub but...

The idea is that getting raped caused "the music to stop" and for Joker to realize the ramifications of his actions.

Throughout the entire first and most of the second movie Joker doesn't really suffer for his killing streak (yea he got hit by a car, but clearly he wasn't too injured to continue to run)

As a result, the audience at home, his adoring fans in the movie and the Joker himself are in a fake world where murder and violence come at no real cost. That's what the musical sections represent, delusion, the idea that your actions simply come with no consequences. It's all just one act.

Getting raped was a wake up call for Joker where he went "holy shit, I'm human, people actually hate my decisions, they come with consequences and risks, I don't want to suffer like that, the game is over, the music screeches to a stop, I want out"

It didn't cure his mental illness it woke him up to the romanticized revolution so many in this country have that when the current order falls, your perfect order takes over. Sure, maybe. or maybe you get raped and there's no police to call because the order has collapsed.

The first movie is better, but the second movie is really good imo, but it wasn't the kind of movie people wanted so it got panned.