Gaffer Tape said: I think his descent into villainy was far more interesting than a Two-Face crime spree would have been. So it just would have been a let down. Besides the fact that his objectives had mostly been accomplished at this point. He was mourning Rachel (oh, and the fact that Harvey actually validated the existence of the character of Rachel Dawes also earned so many points in my book as well) and getting his revenge by giving those responsible a 50/50 chance as well. What else was there for him to do? I think if he had taken the next movie solo, he would have overstayed his welcome....

I agree. He works as a one-film character, with the turn being near the end.