Can you provide a character sketch of Macbeth?

When Shakespeare's Macbeth begins, Macbeth is a noble and courageous military leader. He's defeated the King of Norway and the treacherous Macdonwald in a decisive battle, and for his efforts he is fated to earn King Duncan's approval and admiration. Based on the way the play opens, it's easy to view Macbeth as an honorable man, one who could be potentially destined for greatness.
The way that Macbeth's character unravels is what makes him a tragic figure. With the Weird Sisters' prophetic promises ringing in his ears, Macbeth increasingly allows his ambition to get the better of him. At first, though he is clearly tempted by the chance to kill Duncan and steal the crown, Macbeth seems reluctant to follow through with his schemes. However, once Macbeth actually does murder Duncan, he truly begins to change. He becomes increasingly paranoid and resorts to ruthlessness to satisfy his craving for security. Indeed he even goes so far as to murder one of his closest companions, Banquo, and all of Macduff's family. By the end of the play, Macbeth has become a true tyrant, and, when Macduff kills him, Macbeth seems to have recognized the uselessness of his hunger for power and to have embraced despair. For an example of Macbeth's despair, take a look at his famous soliloquy responding to Lady Macbeth's death in Act 5, Scene 5:

Tomorrow, and tomorrow, and tomorrowCreeps in this petty pace from day to dayTo the last syllable of recorded time;And all our yesterdays have lighted foolsThe way to dusty death. Out, out, brief candle!Life's but a walking shadow, a poor playerThat struts and frets his hour upon the stageAnd then is heard no more. It is a taleTold by an idiot, full of sound and fury,Signifying nothing. (21-30)

This soliloquy is a meditation on the apparent uselessness of life, at least according to Macbeth's perspective. Thus, we can see that Macbeth begins the play as a courageous, even heroic figure, but through the course of the narrative he transforms into a paranoid and despairing tyrant. This transformation is truly tragic, and it's impossible to avoid wondering what kind of man Macbeth might have been if he hadn't heard the Weird Sisters' prophecy.