How is Macbeth a tragic hero?

Macbeth is a tragic hero because he started the play as a good man, but the manipulations of the Weird Sisters and his wife brought out his baser qualities. This leads to Macbeth's moral corruption and downfall by the play's end.  

It is clear Macbeth begins the play as a loyal friend and decent man.  When the Captain speaks about the battle, Macbeth is described as "brave," even as "Valor's minion" (Act I, Scene 2, lines 18, 21).  As a result of Macbeth's great loyalty and service to the crown, Duncan describes him as "valiant" and "worthy" (Act I, Scene 2, line 26).  In addition, Macbeth's wife, the person who would likely know him best, describes him as "full o' th' milk of human kindness" (Act I, Scene 5, line 17).  Macbeth tries to get out of the plan to kill Duncan, telling Lady Macbeth, "We will proceed no further in this business" because his own ambition is not enough to compel him to murder his friend, kinsman, and king (Act I, Scene 7, line 34).

It is not until Lady Macbeth insults Macbeth's pride, deftly manipulating him to do what she wants, that he finally truly commits to their plan.  Lady Macbeth insults her husband's masculinity, calling him a "coward" and saying that, when Macbeth is willing to murder the king, "then [he will be] a man," implying Macbeth wouldn't really be a man unless he goes forward with their plan (Act I, Scene 7, lines 47, 56).

None of this would even be an issue, however, had the Weird Sisters not tried to manipulate Macbeth with their "prophecies."  They say, "Fair is foul, and foul is fair," meaning that good things will look bad and bad things will look good, implying their "prophecies" to Macbeth will seem wonderful but will really lead to terrible consequences (Act I, Scene 1, line 12).  Hecate says she will conjure up "artificial sprites" that will "draw him on to his confusion" (Act III, Scene 5, lines 27, 29); she intends to deceive Macbeth and lead him to his destruction.  Had Macbeth never been subjected to the manipulations of the witches and his wife, he would likely spend his final years peacefully in Glamis or Cawdor.  Instead, before his death, Macbeth finds himself without love, morally bankrupt, loathed by all, and with a name now synonymous with tyranny.