The Batman Who Laughs was created by Scott Snyder and Greg Capullo. He made his first (brief) appearance in Dark Days: The Casting.
Bruce Wayne is the Batman of Earth -22. One night, the Joker binds Batman’s hands and mouth, tells him that it is going to happen tonight. They are near an alley, where a little girl with her parents are walking down an alley. Batman tells him to stop, Joker ignores him and guns down her parents. Moreover, the girl is now “Jokerized”, which makes her laugh repeatedly. All this has caused Batman to cross the line and snap Joker’s neck. Indeed, Batman has killed the Joker, but by doing so, he has saved many people’s lives who’d die presumably.
On the contrary, when the Joker died, Bruce inhaled the nanotoxin that emitted from the joker. Bruce has alerted Clark, Dick, Jason and the others regarding the fact that he’s infected. They tell him that they’ll do whatever they can to save him. Unfortunately, it’s too late, Bruce turns his back and kills all the (former) Robins. From here on, it’s not Batman anymore, but it’s The Batman Who Laughs. Furthermore, he visits the Watchtower and kills the entire Justice League.