The Weird Sisters state in Act 1, Scene 1, of Macbeth, "fair is foul and foul is fair." In what ways is Macbeth both fair and foul?

In many ways, Macbeth begins the play relatively "fair" and ends the play quite "foul."  The Weird Sisters discuss him in Act 1, Scene 1, but the very next time his name is spoken is in reference to his "brave[ry]" because Macbeth

Disdaining Fortune, with his brandished steel,Which smoked with bloody execution,Like Valor's minion, carved out his passageTill he faced [Macdonwald]; Which ne'er shook hands, nor bade farewell to him,Till he unseamed him from the nave to th' chops, And fixed his head upon our battlements.  (1.2.19-25)

This is quite an image: Macbeth resolutely hacking through men, carving a path for himself through their bodies, meeting his enemy, slicing Macdonwald from his belly to his chin, and then plunging a spike through his head to serve as a warning to others.  Duncan, the king, describes Macbeth as "valiant" and quickly decides to honor him with a new title by way of reward: Thane of Cawdor. 

Further, Lady Macbeth, when she receives Macbeth's letter, describes his nature as "full o' th' milk of human kindness" (1.5.17).  She doesn't exactly mean it as a compliment, but readers can understand that Macbeth, prior to and just at the start of the play, was a pretty decent guy.

By the end of the play, however, Macbeth has committed regicide, ordered the killings of Macduff's innocent wife and children while he was away, ordered the killings of Banquo and his son, Fleance, and run the country into the ground.  To Malcolm, Macduff says that

Each new mornNew widows howl, new orphans cry, new sorrowsStrike heaven on the face, that it resoundsAs if it felt with Scotland, and yelled outLike a syllable of dolor.  (4.3.5-9)

Macbeth has, in his greed and dishonor, ruined the country he sacrificed so much -- his self-respect and immortal soul -- to rule.  Macduff says that there is so much sadness in the land that those sorrows hit heaven in the face and cause heaven to cry out as though it sympathized with Scotland.  Even the Weird Sisters now refer to Macbeth as "something wicked" (4.1.45).  He may begin the play pretty "fair," but he certainly ends it utterly "foul."