The whole thing about Batman becoming "lost" is something I've been thinking about, because near the end of the movie - as things get shittier and shittier for Batman by the second - there are some pretty un-subtle hints that he's ready for it, and it's not going to send him off the deep end. I mean, Gordon even says it point-blank: "[We're shitting all over him] Because he can take it." The other big obvious statement is Joker, after being saved from a clean and simple fall to his death, coming to the realization "you're incorruptible, no matter what I do to you."* Blowing up the BOCSATM shows that Batman's not so far into his vigilante role that he's lost his sense of what's not-entirely justified power.
Honestly I think that's one of the things the movie is about - it's about people becoming completely lost and turning into monsters, and it's about other people not becoming completely lost and turning into monsters. This Batman is a little bit different from the others in that he doesn't plan to be Batman until the day he dies - he has a specific goal and a plan, and once he's put Gotham in enough order that he can be satisfied, then he'll hang up the cowl and be Bruce Wayne again (plus or minus one childhood crush). That, and the fact that he has a specific (if bizarre) reason to put on a bat costume and make dramatic entrances all the time, makes him much more mentally stable than the usual "neurotic billionaire with an axe to grind" that starts off sort of lost.
I, uh, I don't have time to gather these thoughts together into a clear point, so you'll just have to figure it out yourself. I think the point was "Nope, still don't need no damned Robin."
* Neat little subversion of The Killing Joke, when you think about it - where this Joker realizes that there is at least one person in the world who won't turn homocidal no matter how bad it gets.