Because I’m capable of it. Note that there will of course be spoilers for the movies.
Inspired by this post: http://airpunchingacademic.tumblr.com/post/29386935683
which asks if anyone else has noticed that Joseph Gordon-Levitt and Heath Ledger look similar.
So, let’s write an insane theory that I’m aware doesn’t make sense. I am making things up primarily to make things up. Now, let us take the whole Nolan universe and turn it on itself.
In the Dark Knight Rises, we see John Blake and see that he’s an orphan and later on he say that his name is Robin, heavily implying that he is Robin. Now, what if we take the whole Nolan universe and turn it on itself.
Since they look so similar, why not theorize that Blake is actually a younger version of the Joker? That doesn’t make sense. The Joker dies before The Dark Knight Rises. So let’s throw in some time travel. Wayne Enterprises, famous for making everything ever and also the nuclear device that threatened to blow up Gotham. We know at the end of The Dark Knight Rises, Robin is given full access to the batcave and (presumably) the technology that Bruce Wayne has been given and has developed on his own. With Bruce Wayne officially dead, there’s no reason for Alfred to hang around to keep an eye on Robin.
So let’s make up a reason for there to be time travel. Say, Bruce Wayne went into hiding for 8 years with regrets about what he had to do to Harvey Dent at the end of The Dark Knight. And he’s always been haunted by what happened to his parents. Maybe Bruce Wayne considered what so many of us would love: some form of time travel (hey maybe he watches Doctor Who). Obviously, it’s never worked for Bruce Wayne, but if you look at it, the Nolan Bruce Wayne relies a lot of his technology and doesn’t tend to do as much of the actual detective work.
Now if we introduce a new character, a younger man, more familiar with the newer technology that Wayne Enterprises and the rest of the world has been releasing, wouldn’t he be more capable? Perhaps he could finish some piece of Wayne Tech that Bruce had hidden away from the rest of the world (ala the end of The Dark Knight).
Time travel is always a tricky thing, so we’ll get to that later, let’s look at Robin/John Blake’s life.
Blake is an orphan on the streets of Gotham, and we can tell that he’s had it rough. Even with the money donated by Wayne, it’s easy to assume a lot of it went toward food and board, meaning Blake would’ve lived a hard life. Possibly looking for food and scraps of clothing so he could save up for other things. Perhaps he learned how to stitch old clothes together. Perhaps he had a knife with which to defend himself when he got older, before he joined the police. And later, while working for Gordon, Blake finds his way to the cement truck (?) driver from earlier in the movie and ends up with both of them dead, albeit by accident. He looks down in horror at the man but seems to recover from it somewhat quickly.
As for how a character like Blake/Robin becomes a character like the Joker, well, there’s plenty of room for him to get his face scarred, but let’s point a bit of the transformation at time travel.
Let’s pretend that time travel has a consequence: memory loss. You wouldn’t notice this if you sent small objects back in time, or maybe even a small animal. Anyone working on a time machine wouldn’t know until they had a proper being with memories walk into the past. Let’s say something happens that causes Blake/Robin to accidentally (or even purposely) test his time machine.
Blake/Robin is in the past. But he’s forgotten things. Important things. But he’s still got some of his skills and instincts. He can sew his own clothes. He can use a knife. He knows how fast a bullet can kill. He’s got scars on his face. No one wants to look at him. He’s a monster. He’s an unknown. There’s no way for him to get back to his time, and he’s forgotten all about his previous (future) life.
This man, lost in the past. Lost in the old Gotham. The one that was nearly torn down by the League of Shadows and Scarecrow’s toxin. He scavenges around the Narrows, trying to get by and he falls on the sidewalk and spots it. A small, rectangular piece of paper. He picks it up and flips it over. A Joker.