I think people are forgetting this is still Batman early in his career. I'm not entirely sure how long the timeline is between Bruce becoming Batman and the events of TDK but I 'm guessing its no more than 2 years tops.
This is still rookie Batman and his gonna make mistakes.
Exactly, which was the whole point of the Joker in the first place. Batman was made, initially, to deal with the mob and regular crooks, the Falcone's and Maroni's of Gotham. The Joker is the embodiment of everything Gordon warned against at the end of BB, Batman should'nt have been prepared, which is why I say that by the end of the next film, his transformation into Batman should be complete, and he becomes the Dark Knight we all know and love from the comics, the one who is prepared and equipped for almost anything.
The Joker said it best, Batman has nothing to threaten him with and nothing to do with all his strength. The Joker doesn't fear death, and wants Batman to kill him to prove his point. He doesn't fear jail, and he isn't after anything other than Batman's own destruction, psychologically. All these reasons are why he is the greatest villain of all time, and why Batman vs the Joker is the most compelling matchup in almost any media, outside of lightsabers. It's a purely mental match, which was captured completely for the first time in a film. Burton touched on it, but not deeply enough, and I also think the way TDK handled it is even better, or at least on par, with the actual comics as well.
It's not about Batman having something to do with the Joker's origin or him falling into chemicals. Batman is responsible for the Joker simply because he doesn't kill him. Bruce's moral code is stronger than anyone the Joker has ever encountered, which intrigues him. Even Frank Miller, who interprets Batman as a Clint Eastwood-style outlaw couldn't allow him to kill the Joker, and I suspect in a weird way, without the Joker, Bruce may become unnecessary, and forced to regain the part of his humanity that died with his parents, something Bruce hasn't tackled since. Batman isn't just some kind of symbol, its Bruce's escape from finally dealing with his demons and moving on with his life. The man is almost obsessive compulsive, without the Joker to chase, how long would he honestly
need to be out there every night? The other criminals are after
something and in someway, can be brought down. The Joker can never be, he's as unreachable as Batman is incorruptible, and in eachother, however small, they see a glimmer of what they could have become had things gone a little differently