you were up late in the Titan's tower with cyborg, making a serum for the joker toxin. Once it was fairly stable and finished you had to test it on someone or die trying. You were going to the most filled area of the infected people in Gotham. There, you found the remains of citizens on the concrete path and standing over them were the remains of the "Batman", or the now batman who laughs.

Bruce: "what a quiet city it is when no one's screaming in terror"

He laughed psychotically