Good thesis statement for the character Macduff in Macbeth.

Macduff serves as a character foil to Macbeth because their contrast conveys with greater clarity Macduff's strengths -- humility, loyalty, and goodness -- and Macbeth's flaws -- excessive ambition, disloyalty, and ruthlessness.  Macduff is not overly ambitious, and he does not seek to increase his power.  When Macbeth is crowned, Macduff doesn't go to the coronation; nor does he attend Macbeth's dinner party later on in the play.  If Macduff were more interested in advancing himself, he could neglect his scruples, as Macbeth has done, and make a bid for greater power.

Further, Macduff is loyal to Duncan, and his loyalty never seems to waver.  When he discovers Duncan's body, he says, "Confusion now hath made his masterpiece. / Most sacrilegious murder hath broke ope / The Lord's anointed temple and stole thence / The life o' th' building" (2.3.76-79).  He doesn't see Duncan's death and Malcolm's flight as an opportunity for himself; instead, he sincerely grieves the death of a good and kind king.

Finally, when Macduff travels to England to see Malcolm, he leaves his wife and children behind, and it doesn't seem to occur to him that Macbeth would direct any violence toward them.  He has honor, something Macbeth lacks.  Macduff initially intends only to unseat Macbeth, but once he learns that Macbeth has murdered his innocent family and servants, he vows to kill the king.  However, it takes a great deal for him to reach this conclusion and it takes relatively little to convince Macbeth to murder.