These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: The Joker serves, in one form or another, as the Big Bad for the firstthreegames, as well as trying to destroy Gotham with a dirty bomb. When he first established himself in Gotham, he forced Black Mask to kill his own girlfriend, then kidnapped and tortured him. After learning Batman won't kill, he strapped himself to an electric chair charged by Bane's heartbeat to force Batman to kill one of them and held Gordon hostage. Over the course of the series, he kills multiple people and tries to kill more (including innocents), sets Bane up to be tortured, freely talks about being physically abusive to Harley Quinn, offers to let his men rape Harley, covers all the usual Bad Boss bases, tries to turn Jim Gordon into a 'roided-up monstrosity, kidnaps Nora Fries to force Mr. Freeze into helping him, blackmails Batman into helping by sending his poisoned blood to hospitals in the Gotham area, and shoots Talia just to hurt Batman. There's also his actions post-mortem with the digital comic, Arkham City: Endgame: He had his minions rig a radio tower within Arkham City to broadcast a message all over Gotham confirming his death, as well as offering a reward of over a million dollars to the person who finds his corpse and deliver it to Harley Quinn, as a way to ensure that the town would "tear itself apart." He also claimed that the walls of Arkham City were still wired with explosives, thus ensuring that the remaining prisoners not only riot within Arkham City in expectation of the walls being blown up, but also, should it have been destroyed, make the rioting in Gotham City even worse with several of the more dangerous inmates participating in wanton destruction. He had Batman lured to the isolation cell that Batman put him in years ago after a particularly heinous massacre he instigated during the Fourth of July where Joker rigged several fireworks with toxins killing hundreds of men, women and children. Finally, he also later revealed that he had Clayface rig the cell, as a means of ensuring that Batman will meet him in Hell.
Evil Is Sexy: If you find yourself having the hots for Harley Quinn in this adaptation, you're not the only one.
Franchise Original Sin: While all the games are generally well reviewed and well received, the prominence of the Joker as really the primary bad guy among everyone else has been criticized, as it lead to the detriment of other villains having a chance at truly taking stage. Origins in particular has a very strong story with or without the Joker being involved, as many fans were excited thinking Black Mask was going to be a more prominent villain than he was.
Harsher in Hindsight: How shall we put it? The Joker himself has been a Death Seeker from the very beginning of the series. In Arkham Origins, he tries to shoot himself in the head in order to end it all, but Batman stops him from doing so for reasons he can't comprehend. In Arkham Asylum, after the Joker says, "I have nothing left to live for," he does shoot himself, but with a Titan formula gun to the throat. His "suicide" is very painfully slow, as it takes a year for the Titan poisoning to course through his bloodstream before his death and its nature complete their course at the end of Arkham City, in fulfillment of his deserving to die as a monster, all the while Batman is unable to intervene with him.