A classic arc, Batman: No Man's Land centered on the aftermath of a cataclysmic earthquake that shook Gotham City to its core. Declared a lost cause and officially cut off from the United States, Gotham quickly descended into crapfest of crime straight out of Escape From New York or uh, The Dark Knight Rises. As you might imagine, this kind of worst-case scenario is basically what Joker masturbates to in the corner of his Arkham Cell, so he wasted no time in kidnapping a hospital's worth of infants.
Sarah Essen, second wife of Comissioner Gordon, found the Joker and dared to confront him alone. But Joker knew that any good person is going to put down their gun if you throw a baby at them...
Could she have let the baby fall and shot Joker, ending his reign of terror and presumably saving thousands of lives in the future? Yes. But Joker knew that you catch a goddamned baby if someone throws it at you, and he took that opportunity to shoot poor Sarah right then and there, leaving the gaggle of babies to wallow in her blood.
The Joker is caught and the babies are saved, because duh, and Commissioner Gordon has an intense confrontation with the man who killed his wife (not to mention the same guy who crippled his daughter). No one could blame this dude for blowing the Joker away. If anyone decided to do this at any point, they'd be doing the world a favor. But Gordon takes the high road, and settles for a good kneecapping.
That's just the thing, though. If Gordon had shot an unarmed Joker, he'd have sacrificed all of his principles and sunk to the level of that scum. Joker wins. But since Gordon didn't go for the kill, Joker is essentially punished with an extended but certainly temporary stay at Casa de Arkham, and will almost certainly go on to kill more innocents. Joker wins again.
No matter what happens, Joker always has the upper hand, and someone else suffers. He enjoys putting people through hell because there's no way he can ever not win. And that's what's so funny.
The Joker and Harley Quinn are without a doubt one of the most revered "couples" in comics, but the fucked up part is that they're really not a couple at all. Relationships are usually founded on mutual adoration and respect; while Harley is infatuated to the point of obsession, her Puddin' has never felt the same way. Any affection Joker does show to Harley is merely a means of manipulating her.
Even in the kindest interpretation of their relationship, Joker is still unable or unwilling to care about Harley. His true love is Batman, and every time she demonstrates an inability to understand that, he punishes her.
Over the years, their relationship has worsened. It was never healthy, but it's come to the point where Joker is never not downright mean and terrible to Harley.
It's not just verbal abuse, either. On several occasions, Joker has taken out his rage on Harley in a physical manner. In one case, he even shot her and left her for dead. Yeah, it was actually Poison Ivy in a Harley disguise, but he didn't know that.
It got even more messed up when the Joker went through his aforementioned facemask phase. The idea being in the rebooted New 52 comic verse, they wanted a nastier, meaner Joker. And that means he gets to punch Harley in the face whenever he feels like it.
It's not fun anymore. This isn't Harley trying to get Joker to "rev her up" and getting pushed over a table. This is just plain terrible. It already resembled a real-life abusive relationship to a startling degree, and since then Joker has taken it up a notch. It wasn't long after her facepunch that Joker shackled up Harley in a pit filled human skeletons, the remains of which he claimed all belonged to previous Harleys. He told her that this is where all of his would-be sidekicks end up, that she's in no way special and has always been expendable.
Joker was probably just messing with Harley here, but that makes it worse. He went through all the trouble of gathering these dead bodies (or making some of them himself), going to Jo-Ann Fabrics and picking out the exact right shade of Harley's clothes, dressing up the skeletons and laying them about and preparing a "torture chamber" to look aged -- all to fuck with his most loyal minion for no other reason than it'd be a good laugh.
Harley's feelings for Joker have wavered from time to time, and she's even left Mistah J a few times, but she always seems to find her way back. One time, Joker got sick of his sidekick and locked her in a rocket and lit the fuse. After escaping, Harley confronted Joker. He simply apologized and all was forgiven.
Joker has killed thousands of people in hundreds of stories, but the fact that he lets Harley Quinn live such a miserable existence counts among the very worst of his deeds.