As much as Batman tries to keep Gotham safe from the Joker, the Dark Knight might actually be helping perpetuate his foe's reign of terror. A threat on Joker's life gave the clown the perfect opportunity to show how he can actually use Batman to his advantage.
For as long as Bruce Wayne has tried to protect the people of his beloved city, the Joker has been around to run amok in it. Whether killing wantonly or engaging in over the top crimes, the Clown Prince of Crime has remained one of the biggest nuisances in the Batman's rogues' gallery. As much as Bruce hates dealing with his oldest nemesis, the hero's strict «no killing» policy means that the only way Batman can handle the Joker is by throwing him back to Arkham.
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But Bruce's commitment to his unwavering moral code is so well-known, even the Joker knows how to weaponize it. In Action Comics #897 by Paul Cornell and Pete Woods, Lex Luthor has come to Arkham in search of an energy source that promises to give Lex a powerful upgrade. Unfortunately, that means Superman's archenemy is forced to interrogate Batman's greatest villain as the energy has emerged within the Joker's cell. With the cameras off and no one around, Lex attempts to have a civilized conversation with the clown, only for the Joker to indulge in his usual trickery. After making a joke at Luthor's expense, the Joker is attacked by Lex who tells him on no uncertain terms that people would be happy if the Joker was found dead. Joker tells his comrade in villainy that if Lex even thinks about making good on his threat then he'd have to deal with Batman, making Luthor relent.
Technically, Lex Luthor and Joker are both right.
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