What is Tom's transformation in Fitzgerald's novel, "The Great Gatsby"?

Tom Buchanan of "The Great Gatsby" is fairly one-dimensional.  In Chapter One, Nick describes Tom as he knew him earlier,

[Tom] had been one of the most powerful ends that ever played football at New Haven--a national figure in a way, one of those men who rech such an acute limited excellence at twenty-one that everything afterwards savours of anti-climax....Tom would drift on forever seeking a little wistfully for the dramtic turbulence of some irrevocable football game.

Now, Nick remarks, Tom has changed and is a "sturdy, straw haired man of thirty with a rather hard mouth and a supercilious manner."  He derogates anyone who does not fit his ideal of the white wealthy class.  And, while he enjoys the excitement of having a mistress, his "supercilious manner" is exhibited in his behavior toward the lower classed Myrtle and her husband.  At the hotel room in New York, he strikes Myrtle across the mouth, and he is demeaning toward George Wilson at his garage.  Later, he manipulates George by playing upon his hatred and jealousy of Gatsby by leading Wilson to believe that Gatsby, as the lover of Myrtle, is responsible for her death--a strategic play in order for him to "win" and save Daisy's reputation. Reminiscent of the football huddle, Daisy and Tom sit at the kitchen table where there "wan an unmistakable air of natural intimacy about the picture and anybody would have said that they were conspiring together" as Nick observes when he spots them in the back of their house.

While Tom Buchanan may have presented the facade of a gentleman because of his wealth and position, he is clearly a brute and a villain, even from the beginning. Dressed in "effeminate ...riding clothes [that] could [not] hide the enormous power of that body,' his initial remarks to Gatsby as he arrogantly points to his stables and other possessions, his further remarks about the state of civilization--"'Civilization's going to pieces,' broke out Tom violently"--establish him as a one-dimensional brute who perceives life strictly as the struggle of the weak vs. the strong: himself and those belonging to him against any who challenge his way of life-- the "irrevocable football game" of which Tom alludes.  In the final chapters, if there is any transformation, it is simply that Tom sheds the veneer of a gentleman.  No longer does he feign any civility as he challenges Gatsby verbally:

'What kind of row are you trying to cause in my house, anyway?....I suppose the latest thing is to sit back and let Mr. Nobody from Nowhere make love to your wife.

With the challenge having been made, Tom later conspires with Daisy and then eliminates his opponent in his brutish way.