Though this is a novel with a moral, the characters in the story are often not involved in moral decision-making. Instead, they are figures of privilege, people who see themselves as being unbound by standard morality. This exceptionalism is fatally challenged in the end, when Myrtle is run over by a car and Gatsby is killed as a result.
The elitism of the rich, in the end, is nothing more than an excuse for bad behavior. It does not protect them (not all of them) from the repercussions of immoral behavior.
Though Daisy, Tom, Jordan and Nick escape punishment for their involvement in Myrtle's death, no characters are finally innocent in the novel. A quick review of the characters shows us how moral (or immoral their world is:
Tom, Daisy, Myrtle, and Gatsby are involved in affairs, cheating in one way or another. George Wilson commits murder. Jordan Baker cheats in her sport. Tom is a racist. Gatsby is a bootlegger and a fraud.
Nick is the only character that grows through the novel. He is not perfect, in any sense, but he matures.
We can see the novel as presenting a progression and development in Nick regarding morality, judgment and empathy. Nick narrates the story and records his responses to the people and events that populate East and West Egg.
His first moral shock comes when he goes to dinner with Tom, Daisy and Jordan and discovers that Tom's affair is a scandal but not a matter of morality. Tom is cheating on Daisy, yet no one suggests that this act is immoral. It is, rather, dramatic and rude, but the affair itself is not as much of an issue as the fact that everyone knows about it.
The concern for perception here is directly related to the shallow values of the rich set; "values" which lead to dishonesty and immoral behavior.
The materialism of the East creates the tragedy of destruction, dishonesty, and fear. No values exist in such an environment.
The example of Jay Gatsby is central to a discussion on the special morality of this group of people. Gatsby puts an great emphasis on achieving a certain dream of love. This dream is, in itself, pure. Yet, to achieve his goal of marrying Daisy, Gatsby must break up a marriage (between Tom and Daisy), effectively undoing two lives in order to realize his own dream.
Gatsby never doubts the validity of his position, morally or otherwise, and maintains confidence even in the end after Myrtle has been run over. He waits for Daisy to call. George Wilson comes instead as an arbiter of misplaced moral justice and Gatsby's dream comes to an end.
Jay Gatsby, the dreamer and romantic, is a liar and a criminal (as a bootlegger) and Nick sees him as being a low sort of person at first. Later, Nick learns to empathize with Gatsby, recognizing Gatsby's rare penchant for maintaining innocence in the face of circumstances that would have erased any innocence in others.
Nick's ability to empathize with Gatsby can be seen as evidence of his own moral growth. In the end, he does not judge Gatsby, but relates to him with sympathy - something he could or would not do at the novel's outset.
Nick is the only character willing to sympathize with Gatsby in the end. He goes so far as to tell Gatsby how he feels, before Gatsby is shot.
[Nick] tells him he is better than the “whole rotten bunch put together.”
This statement represents the sole important moral achievement in the novel. Nick learns that people can be good and bad at the same time and he chooses to see the good in Gatsby.