WARNING: The following contains spoilers for Batman/Catwoman #4 by Tom King, Clay Mann, & Tomeu Morey, on sale now
Batman/Catwoman has slowly teased out a potential future for the Dark Knight where he eventually did marry Selina Kyle and spent the rest of his life with her by his side. After having a child and eventually retiring from active heroics, Batman passed away peacefully. In the aftermath of his death, Catwoman finally embraced her darker side and decided to target some of their old enemies, starting with the Joker.
While Selina killed the Clown Prince of Crime, that act caught the attention of Harley Quinn, who arrived at the door of Wayne Manor in full costume and ready for a fight. And in Batman/Catwoman #4, their throwdown quickly revealed the deepest secrets of both villains turned antiheroes.
Harley and Catwoman are two of the former members of Batman's rogue's gallery still alive in this future Gotham. The two spring into a fight, with Harley angrily yelling at Catwoman for killing the Joker before she ever could. During their fight, Harley expresses genuine shock that Catwoman -- supposedly a good person who eventually even went so far as to marry Gotham's greatest superhero -- could have it in her to commit another murder. However, Selina reveals that she always had that dark edge to her she could never fully erase, saying she was always too bad to be truly good. In turn, she loved Bruce in part because he was such a good man at heart, who despite trying to hide his "heroism behind the psychosis of loss," was always a truly heroic figure in the world.
Selina believes Harley is the same way, calling her "good-girl-gone-bad" origins just an act at heart. At her core, Harley Quinn has never been truly bad. She has too much good in her at the end of the day, and that was what ultimately prevented her from hunting down the Joker and killing him. While she may have had justified rage, she never truly had the killing edge that would allow her to kill the Joker after all these years. To underscore her point, Catwoman says this while also settling any question over which of them is a more skilled fighter by quickly defeating Harley.
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It's a surprising but affecting argument from Selina, harkening to Harley's years of trying to overcome her darker side to become a more heroic person. It's been especially prevalent in recent years, as Harley has transitioned more and more into a genuine ally of the Bat-Family and someone who wants to help fix the parts of the world that the Joker helped shatter in the first place. While this is all nominally happening in a possible future and not the core DC Universe, it's still a compelling argument that suggests that even at her worst Harley Quinn has a central core of good that can't be broken. Although many versions of the character have killed while under coercion of the Joker, this is why Harley can't become the gleeful killer she's often threatened to turn into, and it's why Catwoman could so easily become one despite vowing to her dying husband not to.
It tracks with Harley's restraint in her villainous days, her willingness to befriend others, and plays an important role in her current growth. Despite all her attempts to be bad and now to seem imposing against enemies, Harley is ultimately just too good to be truly bad. But by revealing Harley's greatest secret, Selina hints at her own. No matter how hard Batman tries to help her overcome her past or how many lives she saves, Catwoman isn't a hero at heart. She may prove herself to be a good person again and again -- especially when compared to the other criminals in Gotham City -- but there's always going to be a part of the ruthless thief inside her that prevents her from truly being a good person.