The worst things the Joker has ever done
The Joker, DC's Clown Prince of Crime, is pop culture's most famous villain and has his horrifying reputation for a reason. Over the years, the Joker has fought many superheroes, his arch-rival Batman more so than anyone else. Since he doesn't have superpowers, he resorts to sinister strategies to hurt his enemies. But there are times when his plans are too dark.
His entire relationship with Harley Quinn is toxic and abusive
Every part of his relationship with Harley Quinn can be counted among the Joker's worst actions. While the nature of the relationship is romantic, Joker constantly abuses Harley. Right from the moment when they first meet (when Harley was Joker's psychiatrist), he emotionally manipulates her in the name of affection. Harley's suffering seems more painful because of her refusal to leave his side.
He manipulates Superman into killing his pregnant wife
In the Injustice comic, Joker uses a kryptonite-laced version of the Scarecrow's fear gas on Superman. The gas creates an illusion where everyone around Superman looks like his enemy Doomsday to him. The horrid effects of the gas lead him to fight and kill his pregnant wife Lois Lane. This incident also destroys Metropolis and tips Superman into insanity where he kills the Joker.
He kills Robin with a bomb after beating him brutally
In A Death in the Family, Jason Todd, the second Robin, sets off in search of his mother after Batman asks him to leave him. But the Joker kidnaps Todd. The Joker then proceeds to beat Jason with a crowbar before killing him and his mother with a bomb. However, Jason's death was actually determined by fan polls.
He hands Sarah Essen a brutal death
Commissioner Jim Gordon is Batman's ally, which often makes him the Joker's target. Once when Gordon's second wife, Detective Sarah Essen has the Joker at gunpoint, she finds out that he has held all the infants of Gotham hostage. To catch her off guard, the Joker tosses a child. Sarah tries to catch the baby, leaving her unarmed and the Joker shoots her.
He sexually assaults and paralyzes Barbara Gordon
In Alan Moore's graphic novel The Killing Joke, the Joker shoots Barbara Gordon, which leaves her paralyzed. But that's not all. He proceeds to take obscene photographs of the former Batgirl so that he can break Commissioner Gordon and drive him insane. While his plan fails after Batman's timely intervention, this twisted action really cemented the Joker's terrifying reputation as DC's most prominent villain.