The Joker may be the gold standard for chaotic villains in comic books, but Marvel’s Green Goblin has a record that gives the Clown Prince of Crime a run for his money. In fact, taking into account all the horrible deeds that have been committed by the various madmen who have assumed the mantle, the Goblin may actually have the Joker beat.
Black Cat #4 by Jed MacKay brings back Queen Cat, AKA Lily Hollister, a hero with a complicated history. Before becoming Queen Cat, Lily was dating Harry Osborn, and after stumbling into one of Norman Osborn’s secret chambers, accidentally exposed herself to some of the Goblin Serum, transforming her into Menace, Spider-Man's tragic villain. Menace later allied herself with Norman, who tried to impregnate her with his child in an effort to create more Goblin spawn. Fortunately, the baby was eventually revealed to be Harry’s, not Norman’s, and Spider-Man was able to cure Lily of her Goblin affliction, though the events continue to haunt her to this day.
The effects of Green Goblin’s serum alone are far more dastardly than the Joker’s usual calling cards. While the Clown Prince of Crime typically utilizes a lethal laughing gas, Green Goblin’s serum literally turns people into super-villains, granting them enhanced strength and overwriting their minds with a maniacal persona - not to mention giving them a hideous new appearance. Compared to that, death by laughter doesn’t seem like a bad way to go out. Then there’s all the people they’ve killed; both Joker and Goblin have taken countless innocent lives, especially considering there have been multiple versions of Green Goblin and three different Jokers operating over the years. They've even managed to kill some people very close to their respective nemeses. While Joker killed Jason Todd, Batman’s second Robin, Green Goblin notoriously murdered Gwen Stacy, a worse crime considering Jason was at least a crimefighter, so he was aware of and had accepted the risks of the job. Gwen, on the other hand, was truly just an innocent, killed only so Norman could hurt Peter.
Perhaps the biggest difference between Green Goblin and the Joker is their motivation. The Joker doesn’t really have a motivation beyond making mayhem and toying with Batman, having said on multiple occasions that he never wants their war to end. Green Goblin is certainly chaotic, as well, but Norman also has ambition beyond just anarchy. In Marvel’s Dark Reign series by Brian Michael Bendis, Osborn nearly takes over the world following the Secret Invasion event, creating both a dark version of the Avengers and assembling his own mutant team. Overall, Norman often makes much more strategic bids for power than Joker, who doesn't have much desire to rule or control anything - he's satisfied with simply spreading chaos and wanton destruction. In a way, the Goblin is really a combination of DC's two biggest villains - Norman Osborn is as rich and ambitious as Lex Luthor and as chaotically sadistic as the Joker, making him arguably a greater threat than either of them. His twisted relationship to Spider-Man, having been something of a father figure to Peter growing up, is also more personal, and therefore more impactful, than Joker’s and Batman’s, whose rivalry primarily exists on the streets.
At the end of the day, Batman and Joker and Spider-Man and Green Goblin are probably the two most iconic comic book rivalries of all time. Their relationships mirror one another in a lot of ways, but putting their records side by side, it seems like Green Goblin may actually be the darker villain.