However, I disagree about the ending. It whacked the movie over the head with a theme-shaped bat until its subsequent paste-form could be anything but misinterpreted.

The ending is often misinterpreted but it's extremely good and subtle in a lot of ways. The speech to Murray at the end isn't meant to be an actual explanation of the character's beliefs. It's Arthur breaking down and embracing a role forced upon him. He didn't kill the three people on the subway because they were awful, it was self defense for the first two and the third because his story wouldn't be believed even if it was the truth. A morally good Arthur would still end with him in prison because the third guy would lie and say the evil clown attacked him and his friends.

It's a lot like Joker's laugh. The traditional Joker laughs at the world, finding some macabre humor in the things he does. But in this movie it's just a medical condition. He's forced to do so and people project motivations on to it. He never wanted any of this to happen but society denies him the opportunity to be anything but the villain.