"His referencing a 'truckload of soldiers' getting blown up, his ease with military hardware, and his tactical ingenuity and precision planning all feel like an ex-Special Forces soldier returned stateside and dishing out payback.
"But I just rewatched The Dark Knight, and another wrinkle came to mind about The Joker.
"What if he's not only ex-military, but ex-military intelligence?
"He seems to be very good at the kind of mind-f***ery that sustained, professional interrogation requires. His boast about how "I know the squealers" when he sees one. The way he adjusts his personality and methods depending on who he's talking to, and knowing EXACTLY the reaction he'll get: mocking Gamble's manhood; invoking terror to Brian, the "false" Batman; teasing the policeman's sense of loyalty to his fallen, fellow cops; digging into Gordon's isolation; appealing to Harvey Dent's hunger for 'fairness'," Oswalt writes.
"He even conducts a 'reverse interrogation' with Batman when he's in the box at the police station — wanting to see how 'far' Batman will go, trying to make him break his 'one rule.'
"He constantly changes his backstory (and thus who he is). To Gamble and his henchmen, he's an abused child (figuring that they were also the products of abuse and neglect). To Rachel, he's a man mourning a tragic love — something she's also wrestling with."
Oswalt concluded his theory explaining the way The Joker said his final goodbye to Batman, saying their relationship is "irresistible force meeting an immovable object", is something an interrogator would say after an interrogation.