The Dark Knight

Overview
The Dark Knight follows Bruce Wayne's hope to leave the life of the Batman. Bruce puts everything he has into trying to stop being Batman, and he views Harvey Dent as the one person who can do so. Harvey is one of the few incorruptible people in Gotham, along with Jim Gordon. However, when the Joker starts to wreak havoc in Gotham, things start to change. There is more chaos in general, with many powerful political figures in Gotham being targeted for assassination. However, when the Joker kidnaps both Harvey and Rachel Dawes, with the end result of Rachel's death, Harvey is pushed over the edge. Harvey's change is explained towards the end of the film, when the Joker says, "See madness, as you know, is like gravity...all it takes is a little push." Harvey's change of heart and mind pushes him to harm all of those he blames for Rachel's death, including Jim Gordon and his family at the end of the film.

Love Triangle
One of the large driving forces behind the plot of the film is the love triangle between Bruce, Rachel, and Harvey. It is interesting because Rachel struggles for most of her time in the film to choose who she wants to be with, and ultimately chooses Harvey. However, Bruce does not know she has chosen Harvey. Essentially, Bruce helped Harvey reach the position he was able to reach within the justice system in order to give up life as the Batman. Both had been told by Rachel that they would be together, as she tells Bruce that they would be together if he gave up the Batman and agrees to marry Harvey before her death.

The death of Rachel is used as a tool to show who is truly incorruptible. Harvey breaks when the one he loves has died, going on a rampage for anybody he believes he can place blame on. Meanwhile, Bruce cannot even kill the one who is responsible, the Joker, because of his moral code. Bruce uses Rachel's death as something to push him harder. Bruce's loss makes him fight like someone who has nothing, while Harvey's loss makes him look for payback from people who don't owe him anything. If Harvey would go after the Joker, some could argue that it may be justifiable, but for Harvey to go after other people makes little to no sense.