What does Nick Carraway think of Tom Buchanan?

Nick describes Tom in the initial meeting as appearing arrogant, supercillious, aggressive, strong, and capable of cruelty.

From the text:

He had changed since his New Haven years. 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.

His speaking voice, a gruff husky tenor, added to the impression of fractiousness he conveyed. There was a touch of paternal contempt in it, even toward people he liked—and there were men at New Haven who had hated his guts.

Tom is mean and rude and he doesn't care how rude he is. That's why.