Nightwing knew that something was troubling Batman and the Dark Knight revealed that he was exposed to a new strain of nanotoxin that had been inside the Joker's heart and was released when Joker was killed.
Three days later, the whole Bat Family besides Robin were in the Batcave going through some very vigorous training. Superman mentions how one of the children nearly tore a doctor's throat out, and then Batman laughs, surprising both of them. Labs while Batman returned to the Batcave to begin running some tests. With the GCPD and Arkham Asylum in ruins, Superman had the "Jokerized" children quarantined at S.T.A.R. However, although Joker succeeded in making him break his "one rule", Batman refused to let this turn him into a monster like the clown wanted. Two days later, Batman explained to Superman that the reason Joker went so far was that he was dying from the chemicals that transformed him many years ago. Upon the Joker's death, a nano-toxin began to exit from the villain's mouth, which Batman accidentally breathed in.
THE BATMAN WHO LAUGH FREE
Having had enough and sure that the Joker had earned to die, the Caped Crusader eventually managed to break free of his bonds in a murderous rage and brutally beat the Joker before snapping his neck, killing him. The Clown Prince of Crime killed Batman's entire rogue's gallery, murdered Jim Gordon by dissolving him with acid, set the city alight by blowing up various buildings, and had his goons line up families so he could kill the parents and infected their children with Joker toxin, all in front of Batman. One day, Batman was drugged and captured by the Joker, who went on to destroy Gotham City. While little is detailed about the life of the Batman of Earth -22 before becoming The Batman Who Laughs, it is likely that it followed much the same path as his original counterpart, but his world was always destined to die as part of the Dark Multiverse.