The irony of the title shows, as well as anything else, just how complex people can be.
My interpretation is that Jacob is supposed to mirror Amanda, Hank and Sarah's new born daughter. Jacob is a largely unorganized, simple person. Though we see that he's capable of dark acts, like smashing old men over the head with a tire iron and shooting his friends, those are more actions of instinct then of character and they are always with the idea of saving his brother in mind. He has a high affinity for love and empathy, all human contact treated as valuable. His character is meant to show that the world will take anyone, no matter how innocent, and proverbially shoot them. Jacob gets Hank to kill him because that innocent little baby inside died after seeing two too many people murdered and being told that those are the rules of this world by Hank.
The fox and the crows represent the wild side of man and are placed at the beginning of the film in order to foreshadow the wild deeds that are waiting to take place. They acquire additional meanings as the film goes on, like those mentioned in class today.
Sarah's monologue about not splurging on desert might sound very silly and snobbish of her, but this seemed to be more of a social commentary then a straight speech. She has long accepted her place as a lower middle class wife and mother. When you realize you are trapped in a situation, you go along with it. Such is the case in suburbia, even the quasi-suburbia of this film. Since another theme of this film is that we live in a competitive world, it fits that Sarah's "animal instinct" to live larger than her neighbors is so fierce.
This movie reminds me a bit of Macbeth, here we have a man that is well-enough-off and he stumbles upon a fortune that requires murder. His wife is little eager to pick up the gun herself, but much more so to tell him to do it. At first it is just one murder and they are home free with the prize, but the guilt inside them takes on a disgusting form. Soon, the closet gets overstuffed with skeletons and in the end the main character dies (though just figuratively). In the end, the prize that they desire ends up being in their possession for a very short period of time and they never get to appreciate it. Banquo's ghost does not appear, but the rudimentary theme is the same.
The final scene that Mr. Bennet analyzed with us today got me looking at it in a different angle. In the black hole that is the old window, I see a white blotch in the center, resembling a person from the shoulders up. Simultaneously, the old curtain blows gently out the door, this coupled with the blotch gives me the impression that this is the ghost of Banquo so to speak. Hank and Sarah are haunted nearly every day with their terrible actions and the deaths of all five people they killed. This is the only manifestation of the guilt that we see.