You guys said that Batman "Chooses" to save Harvey instead of Rachel. That is a mistake. The biggest part of Joker's plan was that he knew that Batman would go after Rachel instead of Harvey. He told Batman that Rachel was at the place that, actually Harvey was. That's why Batman screams "Rachel!" When Gordon asks him who he's going after.

So when Batman 'accidentally' saves Harvey instead of Rachel, Harvey loses his mind.

All part of the Joker's plan