Many would argue that there is definitely more than one contender for who is the villain in the opening five chapters of this modern American classic. However, for me, I think there is a strong case for arguing that Tom is the villain of these chapters. This can be seen in the way he keeps a mistress outside of his marriage with Daisy, and how he treats her misogynistically, clearly showing that he believes he is superior to her and that he thinks he can treat both her and anybody else the way he wants to because of his birth and money. Consider the party in Chapter 2 where Tom breaks Myrtle's nose with "a short, deft movement" because of a dispute they have:
Some time toward midnight Tom Buchanan and Mrs. Wilson stood face to face discussing, in impassioned voices, whether Mrs. Wilson had any right to mention Daisy's name.
It is when Myrtle tries to defy Tom's will that she is physically kept down and violently kept in line. Tom is the villain because he feels he can cheat on his wife with impunity, he can cuckold another man because he is of better birth than him and he can violently abuse his mistress if she doesn't do and say exactly what he wants her to do. In response to this, Gatsby's devotion to Daisy looks almost saintly in comparison.