Arkham Asylum: A Serious House on Serious Earth, I liked it because Batman let Harv chose with a flip of the coin. The coin lands good side up, and Face lets him go. Only later do we realise that he made a choice to let him go. With Ra's it was adversarial, the man's trying to destroy the city. With the Joker, Batman regularly debates the morals of killing the wanker. With Two-Face he can't decide, you could say he's of two minds on the subject.
I was thinking more along the lines of "there's still good in him" a la Darth Vader, though. I'm thinking Bats wouldn't condone Harvey's new outlook on Justice and legality, and knows that he must stop Harvey. But at the same time, Harvey's friendship should mean a lot to him and Bats wouldn't want to Jeopardize that. And it makes sense this way to me, because that would continue the trend in Nolan's franchise.
First he learns Justice vs. Revenge.
Then there's Justice vs. Law. (A la the corrupt GCPD)
Then he learns Justice based on hope vs. Justice based on hopelessness - ( a la Ra's Al Ghul.)
Then he learns Justice in the face of Anarchy (A la the Joker).
Then Justice vs. Fear, which should be obvious.
And now he has to know how to stand up for Justice in the face of a friend. Which is one thing in itself, multiply that by two (pun intended), because his friend once believed in that exact same Justice. Hence the Bats / Gordon / Dent relationship. Which would give it weight emotionally, plus he may not be quite out of his mindset on dealing with the Anarchy of the Joker, which might burn whatever Bridge he had with Harvey to begin with. Keep in mind, that Bats' methods for attaining justice should logically be starkly different between the dangerous-explosive-anarchist-theatrical Joker against the idealistic, specific, and former District Attorney Harvey Dent. They're polar opposites.
And even then, I think that would make sense. Gives a little more undercurrent to the whole Duality / Opposites theme that could go on here. Takes Justice vs. Revenge a lot further than what was previously explored.
Does that make sense or did I just blow smoke out of my ass?
This is explained in the movie, when harvey gets scarred in BF he is holding a sheet of paper in his hand and tries to use it to sheild his face as a reflex. However he only manages to sheild one half of his face.
What? Harvey didn't get scarred in BF......... wtf.