WARNING: The following contains spoilers for Batman #99 by James Tynion IV, Jorge Jimenez, Tomeu Morey, & Clayton Cowles on sale now
Batman's rule against killing has been a core ideal of his crime-fighting crusade for decades, with the hero resisting the urge to simply end his enemies and instead bring them to institutions nominally to receive help and guidance. But no villain has tested it more than the Joker. The Clown Prince of Crime has pushed the hero and his allies to the point of murder multiple times over the years, despite Batman's best efforts.
Now, another of his allies is putting the criminal clown into their sights -- Harley Quinn just told Batman that if she gets the chance, she'll end the Joker War by killing Joker. While Batman doesn't approve, he doesn't try to stop her either.
With Gotham burning around them, the Bat-Family has finally been fully reunited. Even Harley Quinn is counted among them, pitching in throughout the event in a bid to stop Joker and Punchline's plans for the city. But while the rest of the Bat-Family are moved into position all around the city to deal with the chaos started by the Joker, Harley decides that she'll be accompanying Batman to ACE Chemical to confront the Joker. Batman refuses -- especially when Harley tells the assembled heroes that she's going there to murder the Joker once and for all. But by the time Batman arrives at ACE Chemicals, Harley has made it too.
She confronts Batman, reminding the hero that despite all his moral posturing, the Joker is always "going to one-up himself." Even if the Joker War is ended and the Clown Prince of Crime ends up back behind bars, the next time he's out he'll just do something even more dark, manic, and terrifying. Harley tells Batman plain and simple that if Batman fails to bring in the villain, or if the Joker kills him -- then Harley is going to shoot Joker down as soon as she can. When Batman tells her no, then Harley matter of factly tells Batman to put her down, and incapacitate her. That's the only way to keep Harley from killing Joker if she gets the opportunity.
Tellingly, Batman doesn't attack her -- either because he has confidence that she won't pull the trigger, or more likely because he agrees with her reasoning. He doesn't knock her out or bind her down or even try to take away the machine gun she's claimed. He just walks into ACE Chemicals, leaving Harley to wait and see if it's Batman or the Joker who walks out of the building.
The Joker War has left Gotham in near-ruins, with countless civilians dead in the chaos. Joker needs to be brought down. Harley might be positioned to deal with him better than anyone else, especially if he actually manages to escape his coming battle with Batman. The question is whether or not Batman will be able to keep Harley from taking a kill-shot -- or if this might finally be the last laugh for the Joker.