A Reason for Nick's Unreliable Narration

Although I agree that Nick’s narration is bias, I believe Fitzgerald has a reason for portraying each character through a negative lens, rather than a neutral or positive lens. I believe Fitzgerald biases Nick’s views in order to show change throughout the novel. Readers like to see dynamic characters; characters that change over the course of the novel, rather than static characters, who are the same no matter what. I do not know how Fitzgerald will choose to show the transformation. He might alter Nick’s views or thoughts on the other characters, showing his change of opinion towards them. However, Fitzgerald might choose to alter the other characters actions thereby changing the way Nick views them.
A perfect example would be Tom. Everything the reader knows about Tom so far, Nick has not agreed with or there has been a negative connotation associated with him. “It’s up to us, who are the dominant race, to watch out or these other races will have control of things.” (p.13) This is one of Tom’s revealing moments, leading the reader to believe he is a bad person. However, Fitzgerald could be showing us the bad in characters only to slowly bring out the good. I am not positive this is correct, but I was trying to bring a different perspective to Fitzgerald’s narration. This is my answer as to why there are negative connotations associated with every major character. My real question is, why is Fitzgerald only showing the “bad” side of characters, especially if they are not going to change? Any other thoughts on why he would do this?