She's pretty sensible about it too, in the sense that she choose the guy who could commit over the guy that dresses up like a bat.

Ah, see, I think what happened with Dent killed that optimistic streak and he realised what he now really needed was for Dent to die since there was no longer a way to save him without destroying Dent's reputation. But also I think Nolan deliberately left that scene open to interpretation by the viewer since we get no clues as to what Batman's thinking in the moment before it occurs.