Shakespeare's Macbeth encompasses the Aristotelian definition of tragedy where the ending of a tragedy causes catharsis, which is an emotional cleansing that is caused by the spectator's sympathy for the tragic hero. The tragic hero, Macbeth, goes through a major character change as the play progresses where Duncan's murder is the first indication of the change. Macbeth murders Duncan because of Lady Macbeth's continual persuasion and the murder psychologically impacts both characters.
In the beginning of the play, Macbeth is displayed as a heroic warrior that fights loyally for Duncan. He is however, tempted by the three witches' prophecy that he will become king of Scotland; this temptation is further driven by the persuasion of Lady Macbeth, who is as ambitious as Macbeth but possess the qualities of malice and evil. Macbeth is indecisive on whether to murder Duncan and is often debating with himself the morality of killing a great king.
He also questions himself why his thoughts lead to evil visions of murdering Duncan.
It is through Lady Macbeth's persuasion that Macbeth murders Duncan, as she chastises Macbeth, "when you durst do it, then you were a man". Lady Macbeth also constructs the plan to kill Duncan, by drugging the guards with alcohol and murdering Duncan in his sleep thereby avoiding suspicion. However, it is not ambition that Macbeth lacks but rather the qualities of malice and evil, which Lady Macbeth possesses, to commit the murder. It is thus evident that Macbeth murders Duncan because of Lady Macbeth's continual persuasion and partly because of his ambition.
After Duncan's murder is committed, Macbeth begins a gradual change in character. Although initially demonstrated as a spineless character that acted because of Lady Macbeth's persuasion and chastisement, Macbeth slowly becomes less dependent on Lady Macbeth. Another change in Macbeth's character is evident when he states that "To crown my thoughts with acts, be it thought and done", which means that he would carry out whatever he thought should be done. This contrasts Macbeth's character in the beginning of the play, where he was in constant debate with himself whether to murder Duncan, yet now he acts decisively and quickly. The assassinations of Macduff and Banquo are also ordered by Macbeth, which further illustrates that Macbeth no longer depends on Lady Macbeth to perform evil deeds.
As the play progresses, Macbeth's mental condition continually deteriorates and he becomes entirely reliant on the witches' prophecy. Macbeth becomes independent of Lady Macbeth and murders without any remorse; he is no longer the heroic warrior evident in the beginning of the play. This character change that Macbeth goes through results in the cathartic ending of his death, which is in accordance with Aristotle's theory of tragedy.