The joker was out on a misson causing trouble for batman and stuff but this time he had a good idea he was gonna act good but than when batman would come and confront him about why he was acting good he'd hurt him bad enough to make him pass out and take his mask off. So that's what he did.

Joker was out in a alley way staring down at his bloody hands Than at batman who was bleeding on the floor as the rain poured down on both of them.