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In my opinion the movie has a really interesting moral undercurrent, and speaks tremendously about the moral code of all human beings and primarily does a good job of showing the conditions of natural law.
(Warning spoiler ahead)
Probably the most interesting scene in my opinion is the scene where in Gotham Harbor are two Ferry boats filled with people. The one Ferry is packed with inmates from the local prison, while the other is filled with innocent citizens. The Joker has devised a ingenious plan whereby both Ferries have been rigged with remote controlled explosives. He notifies each Ferry that they each have a detonator to the other Ferries bomb. However, he gives them an ultimatum. He gives them each 15 min to detonate the other Ferry. Whoever blows up the other Ferry first he will let that Ferry live. If neither Ferry makes a decision to kill the other group of people, then the Joker will blow up both Ferries.
Of course the citizens begin to rationalize that they have a legitimate reason to blow up the prisoners Ferry, because society will be better off if they were to rid all the criminals on that Ferry, that it would be justifiable to "kill the wicked" in exchange for the innocent. However, there are also innocent police officers, prison warden and prison guards to consider who are also on the Prisoner Ferry. This debate goes back and forth for several minutes in the film - showing both sides debating if they should blow up the other Ferry first for their own survival. In the end, neither group could bring themselves to blow up the other Ferry.
What came to mind was immediately natural law. Instinctively, each person - both criminal and innocent citizen realized to kill 300+ people would be a monstrous act and neither could allow that thought to pass by their conscience without tremendous guilt and shame. This also reminded me of when Jesus said to the group ready to stone the adulteress - "You without sin cast the first stone". It also reinforced to me that mankind is not "Totally Depraved", but man does operate according to his conscience and natural law instilled in us by our Creator. If man were truly "Totally Depraved", then by the very totality of that depravity man would have had no problem pushing that detonator - nor any associated guilt in doing, as guilt is a direct outward working of natural law. It would have been like child's play - simple, no guilt, no remorse. But instead - mankind has tremendous potential for good. In the movie the Joker is the one who thought man was totally depraved - was inherently evil. The Joker is trying to show Gotham that they need not to live by a moral code, but they should live according to their "true nature" - evil and chaos. It is the hero or the savior of the movie, Batman shows Gotham that he believes in the good people of his city and that they have great potential for good. He believes that given the right motivation they can rise above evil and live in peace. He is willing to go to any lengths to do this for the people of Gotham.
Again, I see many parallels between the redemptive story of Christ and his desire for his children to choose love over self. That Christ sees the great potential all of his children to love the Father and our neighbor. Christ came to awaken us from the depths of our own selfishness by dying on a cross. That he would go to great lengths to show us he desires mercy over sacrifice, that he desires genuine fellowship with his creation. It is a story that is universal and should speak to us all.