Lust- Lust is defeated by Mustang. She symbolizes Mustang's lust for power. Lust paralyzed Jean Havoc, another friend and follower of Mustang, so she symbolizes how his lust for power harms his friends. Hawkeye, present during envy's death, is a key factor, because she loves him and wants to help him achieve his goals, without becoming obsessed with them. By killing Lust, Mustang is able to conquer a part of his personal deadly desire.
Gluttony- Gluttony is eaten by his older brother, Pride, in an ironic twist. It symbolizes how overindulgence can eventually kill you. Gluttony's symbolism isn't very important, but next up, is Pride.
Pride- Pride is defeated through his battle with Ed. This fight represents Ed's pride of achieving the title of being a state alchemist. While he is helped by Kimblee, it is because Pride is being weakened through the fight by Ed's desire to overcome him (symbolically). He essentially destroys himself by deciding Ed, a human whom he despises, should become his next vessel. Ed defeats him, causing him to revert to his true form, a tiny baby. That too, shows the weakness of Pride.
Envy- Envy is defeated by Mustang; however he is the only homunculus to commit suicide. Envy is responsible for the death of Hughes, another example of how Mustang inadvertently causes his friends to meet undesirable ends through his lust for the seat of Fuhrer. Envy is also the source of Mustang's Wrath (who is still alive) and represents another "sin" of Mustang. (He's pretty bad, he has 3!) He envies the power of King Bradley. While his intentions for Amestris are good, he has some refining to do, as pointed out by Scar and Ed. In the end, Envy kills himself, and another sin of Mustang is lessened.
Wrath- Wrath is killed by Scar; however the battle does take a toll on him too. The defeat of the King has a double meaning. One, it literally Scar killing the object of his anger, the man responsible for the destruction of Ishval. Two, he is figuratively quenching his wrath and will cease to be a wanted serial killer. It is a life altering event for Scar.
Sloth- sloth is killed by the Armstrong siblings. While the two aren't inherently lazy, the two do mention that they won't sit idly by while the Elrics do the fighting.
Greed(Ling)- Greed is killed twice by his Father, but for this example, we will go with Ling. When Greed dies, helping Ed to land a lethal blow on Father, he is released from Ling, who previously had told him that material possessions weren't what he desired. What he truly desired were friends. Greed accepts this as he dies and Ling comes to the same conclusion that he shouldn't become emperor for the riches and power, but solely for the people. He realizes this far before Greed does, and passes it on.