So it just occured to me while watching the film during school: Harvey pretty much had to die. Had the city known what happened to him they'd have lost it, I think. Going down the hero allowed for the city's hope to remain. All Batsy lost was the law. The people have always thought him a madman so his reputation with them was hardly defiled. Considering the condition the city was mentioned to be in it's quite obvious--I think--that had Dent not played such a crucial roll--and I'm just hypothesizing what the filmmakers meant--that man on the civilian boat would have killed the prisoners.