Tom Buchanan prevents Jay Gatsby from "living happily ever after," both in Gatsby’s head (for much of the story) and then literally (by denying him Daisy and then taking actions that lead to Gatsby’s death). Nick introduces Tom Buchanan as an excellent sportsman, but that’s about the only thing he has going for him. Tom is wealthy, restless, and cruel – not a good combination. Fitzgerald gets us to dislike – if not flat out hate – Tom, so that we root for Gatsby. Tom’s violent and abusive nature certainly contributes to our emotions.
But if you like your antagonists cut out in clear black-and-white, Tom is not the guy for you. Just when we feel safe about sticking Tom in the villain box and sealing the lid shut, we start to (gasp) feel sympathy for him. At the Plaza, for the first time we see a sensitive side to Tom. Or, as Nick says it, the man displays "a husky tenderness" towards his wife. He declares he loves her, that he’s always loved her, mentions some tear-jerking scenes between them, and declares he’s going to treat her better from now on. By the end of the novel, even Nick is won over – at least a little bit. He declares that he can’t forgive Tom for what he’s done, but that he certainly understands that, in Tom’s mind, all his actions were justified. So there you have it: Tom Buchanan, the not-so-evil bad guy.
Alternatively, Gatsby’s antagonist could be seen as the "dream" he has to be with Daisy. This is the dream that has driven his actions, propelled him forward, and, ultimately, clouded his judgment in a way that led to his death. Gatsby can never come to grips with the reality of the present because he is stuck in his dreams of the past.