When Sansón Carrasco first steps onto the scene, we get the feeling that we're not going to like him. The narrator describes him by saying he is "one that would delight in nothing more than making sport for himself, by ridiculing others; as he plainly discovered when he saw Don Quixote" (184.108.40.206). Sansón, in fact, is the person who eggs Don Quixote on to go to the tournaments in Zaragoza and to resume his life of knighthood.
But from that point onward, the book seems to change its attitude toward Sansón. As Part 2 of the book unfolds, Sansón seems to be the main guy who's trying to cure Don Quixote of his terrible affliction. He dresses up as the Knight of the Mirrors in hopes of defeating Don Quixote in combat and forcing him to return home. But when he's beaten in their first match, Sansón swears revenge: "[I]t is a folly to think I ever will go home, till I have swingingly paid that unaccountable madman […] it is pure revenge now" (220.127.116.11).
Sansón's fancy education has made him kind of cocky and kind of a jerk, but he learns from his mistakes and ends up helping Don Quixote. After he has finally defeated the Don, Sansón says that he hopes "that the honest gentleman, who is naturally a man of good parts, may recover his understanding" (18.104.22.168).
Like other characters in the book, Sansón isn't purely one thing or another. It's hard to tell if he's good or bad, and that's probably the way Cervantes wanted it. Sansón is the closest thing the novel has to an antagonist, aside from the Don's books themselves, but even he is never all bad. It's fitting for a novel in which everyone shows off both bad and good qualities that the villain, such as he is, turns out to be a kind of chill in the end.
(Note: Sansón Carrasco's name appears in some editions as "Samson Carrasco" or "Sampson Carrasco.")