Why This Is The Only Joker Origin Theory That Works
Moreso than any in-movie evidence, what makes this theory so captivating is that while it may seem to limit the unending possibilities of the ambiguous backstory, it actually strengthens the mythology of the Joker. Here's a character that, by this point in time, it's clear no definitive version can exist. Caesar Romero, Jack Nicholson, Heath Ledger and Jared Leto have all provided their own takes in live-action to varying responses, but even the ones contemporarily viewed as unbeatable (Nicholson and Ledger) are impossible to cite as fully embodying the full scope of the character as written. Mark Hamill's take from Batman: The Animated Series comes close, but even that's a product of its time and medium.
And that's kind of the point. Just as Batman is more symbol than man (a key, overriding theme of The Dark Knight Trilogy), the Joker is more than just a crazed asylum escapee. He's not one disgruntled citizen, but a representation of everything that the hero opposes. And the soldier, Bush-era Joker that Ledger seems to embody is the essential foil to Christian Bale's Bruce Wayne. To have the Man Who Laughs or a deranged gangster would be fitting in terms of source, yes, but when it comes to reflecting the times and capturing a modern audience, the 2008 world begged for something different.
This reading makes him a more explicit product of the very grounded world Nolan created, and even more a dominant pivot in the story. After all, while Heath Ledger is undoubtedly the MVP of The Dark Knight, the decade-long discussion on the role and its legacy often misses that - like Hannibal Lecter and Darth Vader before him - these villains exist primarily to reflect on the protagonist's journey. Batman is taking on the ills of modern America and discovering that his moral code cannot match; via the corruption of Harvey Dent, it becomes clear that things can't be simply reset; the unstoppable force the Joker represents is too powerful.
The movie ends with Batman going into hiding, an attempt to break the cycle of escalation, but one wrought with sacrifice. It's impactful on a character level, but utterly shaking when read within the wider global context. The Dark Knight explores new battlegrounds, and having the real world parallel makes the message all the more pointed and real.
Ten years on, the notion that the Joker is a PTSD-suffering former-soldier is now widely known and, while no theorized origin for such an unknowable character can ever be resolutely confirmed, is undeniably the best. It not only gives logic (however unnerving) to his actions, it ties into the greater themes of The Dark Knight Trilogy and shapes how we read what it does to Batman.