The following contains spoilers for The Joker: The Man Who Stopped Laughing #4, on sale now from DC Comics.
The Joker has committed several horrendous crimes, with these actions specifically targeted at both Batman and the civilians of Gotham City. However, his arguably worst act involved luring Jason Todd into a trap and killing him, something that had a huge impact on the Batman mythos for years. Strangely enough, this moment seemingly meant very little to the Clown Prince of Crime.
In The Man Who Stopped Laughing #4 (by Matthew Rosenberg, Carmine Di Giandomenico, Romulo Fajardo Jr., Arif Prianto, and Tom Napolitano), the Joker and Red Hood come to blows once more. The Joker gets the upper hand but doesn't kill Jason, he instead makes a disturbing statement concerning his unusual act of mercy. This makes the Batman villain seem even more evil than ever before, as it showcases both his sadistic sense of humor and how little he cares about human life.
The Joker had a Chance to Kill Red Hood - Again
In The Man Who Stopped Laughing #4, The Joker is running rampant in a hospital, spending time to creepily entertain a group of sick kids while he's there. When he tries to make his escape, both the authorities and the violent Red Hood head toward him, setting up a twisted bit of irony. Red Hood was the resurrected Jason Todd, the second Robin who the Joker had killed. Having been back as Red Hood for years, Jason is well-known for his violent and murderous ways.
Despite having no qualms about killing, Batman's former sidekick finds himself at the Joker's mercy. He spends the time questioning the clown's motives, namely how he was still alive when he had supposedly died. The Joker laughs about Red Hood's "immortality" before leaving him be. Jason is shocked that the killer clown didn't try to kill him again, but The Joker's response strikes just as hard. As he leaves the former Boy Wonder alive, he claims that he never wants to make the same joke twice.
"A Death In the Family" was a watershed storyline for Batman and comics in general. Robin had long since become a somewhat campy character, being largely associated with the Adam West Batman TV series even as the Caped Crusader himself had his image darkened. Still, the Boy Wonder was an iconic superhero, so having Batman's main villain kill him in a particularly violent fashion showcased just how grim and gritty comic books were getting. For years, Batman was haunted by these events, and it wouldn't be until several years later that he officially took on Tim Drake as his new Robin.
Jason Todd's Murder in "A Death in the Family" Meant Nothing to the Joker
Nevertheless, the trauma for both Batman and the real world over Robin's murder was just a fleeting moment for the Joker, who refers to the death as a joke. This says two things about him, neither of which puts him in a good light. For one, his sadism knows no bounds, with human life only existing for him to use in a morbidly entertaining way. This is why he can randomly kill people without any sort of remorse.
Likewise, this misanthropy is amplified when it comes to Batman, with the Joker using all manner of "gags" to get to the Dark Knight. It echoes the events of Batman: The Killing Joke, namely its ending involving Batman laughing at a joke. Thus, The Joker truly is Batman without any sort of moral code and stripped of his humanity. He is dedicated only to the "mission," the likes of which involves murdering or torturing everyone in his path. Unfortunately for Jason, he was just one of several unwitting actors in the clown's antisocial plays, and the joke was ultimately on him. Whether it's smiling fish or mass murder, everything is a joke to the Joker, though he's usually the only one laughing.