Aristotle defined a tragic hero as a character who errs in judgment, the result of some tragic flaw, and this error leads to his own destruction. The tragic hero usually starts from a place of relative goodness, as Macbeth does. He begins the play, described as "brave" and "valiant" (1.2.18, 1.2.26). His wife, arguably the person who knows him best, believes that he is "full o' th' milk of human kindness" (1.5.17). Macbeth's error is murdering his king, cousin, and friend, Duncan, as a result of his own tragic flaw: pride. When he considers his one reason to commit the murder, his "Vaulting ambition," he actually decides to "proceed no further" in the business of killing him (1.7.27, 1.7.34). It isn't until his wife wounds his pride, calling him a "coward" and claiming that she would never be so disloyal as to go back on something she had "sworn as [he] / Had done to this" that he relents and determines to move forward with the plot to commit regicide (1.7.47, 1.7.66-67). The tragedy is that a good man falls, becoming evil. If Macbeth were a monster to begin with, then killing the king (and everyone else) would only be in keeping with his character all along. Therefore, he is a tragic hero.