I think most of the characters were vampires because they fed off other people.
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It is not just that they feed off others, but that there is no remorse. They take life and bring none of there own. The most obvious example is Tom. He dangles a car in front of Wilson, while having an affair with his wife. This later leads to her death, which he allows Gatsby to take the fall for, even though he knows the truth. The lower classes in general mean nothing to him. They are just there for him to feed on. Jordan also, but to a lesser extent. Jordan drifts from home to home, using people and resources without care. She also has no sympathy for the lower classes, evidenced by her desire to get something to eat after they encounter the deadly accident.