Inside the plant, the heist goes wrong. Batman appears. The terrified Red Hood jumps into a vat of chemical waste to escape, only to be flushed out into a drainage basin. When he pulls off the mask, he looks into a mirror—and sees the Joker for the first time: bleached-white skin, ruby-red lips, green hair. His "one bad day" has physically and mentally unmade him.
Alan Moore himself has expressed regret over the violence done to Barbara, calling it "shallow" and "clumsy" in retrospect. "I made it too cruel," he said in a later interview. "I wouldn't write it that way now." Batman- The Killing Joke
Moore was approached to write a Joker story. Initially reluctant, he was intrigued by the idea of giving the Joker a definitive origin—something that had only been hinted at in past comics (most notably in 1951’s "The Man Behind the Red Hood!" by Bill Finger and Lew Sayre Schwartz). Moore’s concept was bleakly simple: to explore the thesis that anyone, even the most upright citizen, is just "one bad day" away from complete insanity. Inside the plant, the heist goes wrong