If there is an antagonist or a villain in the novel, it is Tom. He is racist and he generally mistreats everyone he comes into contact with. Tom loves Daisy but he treats her more like a possession than a partner. He is having an affair with Myrtle. During an apartment party with Nick and Myrtle's friends, Tom breaks Myrtle's nose for having mentioned Daisy's name. Tom is also condescending to George, Myrtle's husband. Tom also seizes the opportunity to implicate Gatsby in Myrtle's death, thereby indirectly leading to Gatsby's and (more indirectly) to George's deaths. Tom comes from money and is proud of his heritage.
Gatsby comes from a poor family and has made it his life's work to achieve money and success, to distance himself from his heritage. This is one of the many ways he is different from Tom, who came from money. While Tom is the villain of the novel, Gatsby (and to some extent, Nick) is the hero.
One thing Gatsby and Tom have in common is their infatuation with money. Gatsby got a taste for wealthy life when he met Dan Cody and his strategy for winning Daisy back, years later, also necessitated (in Gatsby's mind) the accumulation of more wealth. While Gatsby's pursuit of Daisy is conflated with monetary and material value, there is a degree of love and idealism involved. This is what sets him apart from Tom again. Although Tom loves Daisy, it seems that Gatsby's love is more idealistic, more passionate, and therefore it is perhaps more genuine.
It is difficult to find any redeeming qualities with Tom. Gatsby is an admirable, idealistic hero but he does have his flaws which include engaging in illegal activities. We might also criticize Gatsby for disrupting Tom and Daisy's marriage. Even though he does this for love, his actions eventually lead to disaster. However, Tom is equally if not more culpable for the tragic events that unfold. It is not simply put that Tom is bad and Gatsby is good but for the most part, Gatsby is our flawed but idealistic hero and Tom is the villain.
Note how Nick's perspective of Tom and Gatsby helps to inform the reader. Nick initially despises Tom and this increases as the novel goes on. Nick initially likes Gatsby but does not approve of certain aspects of his lifestyle. But by the novel's end, Nick's admiration and appreciation for Gatsby has grown. Again, we see two different trajectories. Nick's scorn for Tom increases over time and Nick's affection for Gatsby increases over time. In Chapter 8, Nick realizes that he does find Gatsby to be more genuine than Tom and all the others in that circle of high society:
"They're a rotten crowd," I shouted across the lawn. "You're worth the whole damn bunch put together."
I've always been glad I said that. It was the only compliment I ever gave him, because I disapproved of him from beginning to end. First he nodded politely, and then his face broke into that radiant and understanding smile, as if we'd been in ecstatic cahoots on that fact all the time.
Gatsby is the defacto hero of Fitzgerald's novel and Tom is the villian. The two men come into conflict over Daisy, a woman they both wish to claim for their own.
Tom is a bigoted and brutal man, repeatedly identified by his physical characteristics. He is “one of the most powerful ends that ever played football” at Yale and is often described in his square-shouldered, intimidating physicality.
Tom's moral character is evidenced by the things he does - he hits Myrtle; he cheats on his wife; he strings along George Wilson giving the man hope then leading him to murder Jay Gatsby.
Importantly, Tom is "old money". He was raised in a very wealthy family and takes his wealth entirely for granted (along with the privileges and power that accompany that wealth). Gatsby is new money. Gatsby has built everything he has on his own, rising from relative poverty to great riches.
Gatsby stands in contrast to Tom's hardened cynicism. Gatsby is a romantic and a dreamer who believes that dreams can come true - at least his own dreams. Where Tom is shaped by the world that he was raised in, Gatsby is a man made of his own projections, his hopes and dreams.
“The truth was that Gatsby of West Egg, Long Island, sprang from his Platonic conception of himself.”
The hope associated with Gatsby helps to mitigate some of his own questionable decisions and immoral behavior. Gatsby, like Tom, views Daisy as a kind of wealth. She symbolizes success for them in ways that objectify and diminish her actual value as a person. Gatsby is not innocent of such short-sighted thinking, but his vision is driven by a romantic impulse where Tom's stems from a sense of entitlement.
For Gatsby, Daisy is a final jewel to be won to make his journey complete. For Tom, he feels that Daisy is simply what is owed him.