The Joker is known as one of DC Comics' most sadistic killers, but when he was forced to face his crimes in a psychically-induced moment of clarity, the Clown Prince of Crime's reaction was surprisingly heartfelt.
Batman's eternal foe has earned himself a reputation as a ruthless killer, and rightfully so. The Joker has killed a lot of people, easily in the thousands by his own estimate. And while his body count does make him a standout killer, it's his methods that truly set him apart from the rest of Batman's gallery of rogues. Whether it be the cold-blooded murder of Jason Todd, a slow death brought on by his signature Joker Venom, or blowing up a school full of children, the mysterious and deranged supervillain has done it all. When he was finally forced to confront his crimes, however, the Joker was reduced to a remorseful, sobbing mess.
In 2003's JLA #84, by Joe Kelly, Doug Mahnke and Tom Nguyen, a mysterious force sweeps across the globe, instigating a wave of remorse among the Justice League's cavalcade of villains. Many of DC's worst criminals begin not only confessing their crimes, but begging for death as penance for their misdeeds. Even Lex Luthor, then-President of the United States, is left in a catatonic state in the wake of this forced remorse. To investigate the strange phenomenon, Batman makes his way to Arkham Asylum, where he finds a room full of his most despicable rogues reckoning with their deeds. When Bruce lays eyes on the Joker, he can't help but smile at the Harlequin of Hate's reaction to being cursed with empathy.
In this moment, fans get a glimpse at what is hinted to be the Joker's first clear-headed reaction to his laundry list of crimes, and it's likely that if that reaction were ever seen by the rest of Gotham's criminal underbelly, it would put an end to his reign as the Clown Prince of Crime. Joker has built his criminal reputation around his love of senseless violence, and the glee he shows in the face of horror is what truly makes him such a terrifying villain. Many of Batman's other rogues show remorse - even if feigned - for the destruction they cause, and some, including Harley Quinn have even switched sides to make up for their misdeeds. The Joker, however, freely admits he kills and destroys for his own enjoyment. Were the other residents of Arkham not suffering from the same fate at the time, it would have been enough to destroy his reputation.
The change happens because of the influence of a Martian telepath bringing out Joker's more human side, and it's not the first time this power has been used to expose Joker's secrets. The Martian Manhunter once revealed to Batman that deep within his mind, Joker still wants to be safe and loved. While the Joker's horrific outer personality is just as real as these feelings, his capacity for normal human emotion would likely shock his criminal allies and civilian victims alike.
Joker has earned the respect (or, more often, fear) of his fellow supervillains by being an unpredictable menace, killing in disturbingly creative ways often for personal amusement. While it's made Joker a notoriously terrible partner in crime, it has also made him one of the most respected villains in Gotham and beyond. Even Batman was surprised to learn that deep down, Joker has the capacity for remorse - a reaction which would obliterate his demonic reputation if it ever came to light.