Unfortunately, the Joker had miscalculated.
Schiller's human facade was even more perfect than he had imagined. He had actually begun worrying, like some utterly commonplace ordinary person, about the impact a major bank robbery might have on housing prices in the surrounding neighborhoods.
The Joker had come to find Schiller today because he knew that even if Schiller knew what he was planning, he would not stop him. After all, Schiller truly did not care how many people died.
As long as the Joker told him in advance and Schiller failed to stop him, then when the chain reaction came back to haunt Schiller, he would have no one to blame but himself.
After all, they had told you beforehand. If you had stopped it, wouldn't none of this have happened?
The Joker often used this trick on Batman too. The evil was clearly his doing, but Batman learned about it beforehand and failed to stop it because of one selfish interest or another, one personal feeling or another. Afterwards, Batman would doubt himself.
The Joker understood very well how to shake a person, because he knew what everyone wanted most. Schiller was not adjusting well to his current life. He felt anxious, so the Joker offered him an automatic clear-all solution. Schiller had no morals anyway. He could solve every problem without sacrificing anything. Who would choose no?
But Schiller chose "or" between "yes or no." He would not stop the bank robbery for moral reasons. He would stop it for housing prices.
That disgusted the Joker to no end.
He could understand a hero stopping him for some childish idealism. He could understand a cop stopping him out of duty. But he absolutely could not stand an ordinary person stopping him for money.
Because if that happened, the