Tom Buchanan is the clear villain in F. Scott Fitzgerald's The Great Gatsby, and it is difficult for readers to feel any sympathy at all for him because he is such a self-absorbed and careless person who causes destruction everywhere he goes.
We see him first in his own home, and Fitzgerald makes his characterization clear in this description:
Now he was a sturdy, straw haired man of thirty with a rather hard mouth and a supercilious manner. Two shining, arrogant eyes had established dominance over his face and gave him the appearance of always leaning aggressively forward. Not even the effeminate swank of his riding clothes could hide the enormous power of that body--he seemed to fill those glistening boots until he strained the top lacing and you could see a great pack of muscle shifting when his shoulder moved under his thin coat. It was a body capable of enormous leverage--a cruel body.
Note the descriptors: hard, supercilious, arrogant, dominance, aggressively, enormous power, strained, muscle, enormous leverage, cruel. Despite his rather feminine-looking riding pants and his tenor voice, Tom is the picture of a bully.
As we spend some time with Tom, we discover he is also a despicable and morally bankrupt character. He is a racist who believes that the white race is superior, and he clearly believes that he is above or beyond every rule. He has cheated on Daisy, his wife, for their entire marriage, as early as their honeymoon. He is violent to women, as evidenced by Daisy's bruised knuckle and Myrtle's bloody nose. He is having an affair with Myrtle and scorns her husband by conducting the affair right under his nose; even worse, he does not even attempt to hide his adultery from Daisy. We know this is a pattern because the Buchanans have to move often to avoid the scandals his cheating causes.
Though he mocks Gatsby for working to earn his money, Tom uses his unearned money to please himself--and to get himself out of trouble, of course. He does not particularly want Daisy, but he is incensed when someone else wants her and will really love her. She is his possession, and he does not like to lose what is his.
Tom and Daisy are "careless people," and their worst carelessness happens at the end. Daisy hits Myrtle with the car but does not stop; in fact, she lets Gatsby take the blame for it. Even worse, Tom knows what the rather deranged-by-grief George Wilson wants to do if he finds Gatsby, and he gladly gives Wilson directions to Gatsby's house so Wilson can kill him--and himself. All four characters who die in this novel do so because of Tom Buchanan.
While Tom is not a criminal, he is a despicable character who possesses no redeeming qualities. None.