It seems that for all of his bluster, the Joker may have been secretly intimidated by and jealous of Harley Quinn. The Clown Prince of Crime revealed his true feelings the moment he saw his sidekick show genuine potential as a solo villain.
For the majority of his career, the Joker's been a solo act who rose to fame with his one-of-a-kind gimmick and insatiable devotion to destroying Batman. And while the giggling villain has occasionally taken on swaths of henchmen over the years, there's really only one person who could ever be called his 'partner:' his former psychiatrist and former girlfriend, Harley Quinn. After using his charm and wit to lure her away from a strait-laced career at Arkham Asylum, Harley joined the Joker as his sidekick. Unlike the numerous nameless minions he'd employed over the years, Harley had skills and a tenacity that kept her in the Joker's orbit for years.
Related: Joker's Life After Harley Quinn Exposes the Dark Truth of Their 'Love'
And all the time working for the Joker, Harley was apparently holding back on what she was actually capable of. In Harley Quinn #1 by Karl Kesel and Terry Dodson, Harley has just broken Joker out of Arkham and returned him back to their hideout to recuperate. After the Joker discovers that an amusement park owner has stolen his likeness for a theme park show and ride, Joker decides to take revenge on the businessman. But with Harley demanding the Joker's attention non-stop, he puts his second-in-command in charge of orchestrating the plan just so he can get some space. Initially, Joker and his gang share a few laughs at Harley's attempts to come up with a plan, but as the week goes on, even Joker's henchmen agree that Quinn has a lot of great ideas. This sends the Joker into a jealous fury.
It's a rare sight to see the Joker express any kind of emotion, much less jealousy. The villain's worldview is so warped that he generally can't help but find the humor in even the worst of situations. But potentially being outdone by Harley is something he clearly can't handle by. In fact, this intense triggering of Joker may explain why he spent the most of him and Harley Quinn's relationship berating her and treating her as poorly as he did.
When Joker drafted Harley to join him in his misadventures, he wasn't looking for an equal partner, he just wanted another expendable lackey that would obey his commands. But Harley Quinn had a resiliency that made Joker keep her around far longer than any nameless henchman because she could clearly add more to the Joker's operation that just another set of hands. But Joker is the type to not look for any improvements to his methods. Whenever Harley began showing potential, he shut it down to try and crush her spirits and keep himself on top. Joker's jealousy isn't a good look, but it definitely explains more than a few things about his cruelty towards Harley Quinn.