There is much in the character of Macbeth that is puzzling. While not all are fans of the literary concept of ambiguity in general as it is thought to be overused, it is true that there is much about Macbeth's character that is ambiguous and isn't readily discernible from the text or story events. At the start of the play Macbeth, Macbeth is a loyal follower of King Duncan and has just fought at his side to defeat an enemy of Scotland. Macbeth is the most admired and most skilled leader and nobleman in Duncan's army and all expect that he will be named as successor to Duncan.
Macbeth is set in a time in Scotland when royal succession was a matter of the king hand picking a successor or the eligible nobles fighting to the death until one was left--not very efficient use of noble blood. Duncan decides to pass over Macbeth not because he is unworthy or has questionable loyalty but because Duncan seeks to establish the stabalizing rule of male primogeniture: the crown going to the eldest male descendant. So while Macbeth is a great warrior, a great nobleman, a great military leader, and a great and loyal liege to King Duncan, overriding concerns of greater importance excluded him from succession.
Macbeth reacts, as do his comrades, with normal shock and disappointment and undoubtedly some self-righteous self-congratulatory rage at Duncan's choice but his reaction extends a little further than his comrades because he has dreams of ultimate power. Now his character sketch gets a little tricky. Why does he believe the Witches? Why does he let Lady Macbeth insinuate and insult him into doing what his conscience naturally recoils against? Why does he proceed with the deed even though his conscience, some would say his soul, is crying out against carrying it forward? Does he really want the power or is he motivated by some other more complex psychological reason? Does he go mad and when? Does he have any traits that permit him to be read sympathetically and forgiven? Even Banquo wants to know, "why do you start, and seem to fear / Things that do sound so fair?" (I.iii.51-52).
It may be that Macbeth is superstitious while being a powerful leader and general who is notwithstanding weak willed and full of conceit and vain egoism. These traits would explain why he caves in to the witches and to Lady Macbeth, who manipulates him by assaulting his manly qualities of courage and fortitude. Perhaps his weakness is that he, though a domineering leader of armies, is easily dominated himself and thus incapable of attending to his own conscience even though it speaks to him and recoils against doing the wrong prompted by others. Perhaps this is why the Witches approach him in the first place: they perceive the cowering spirit in him that underlies the greatness on the field of battle. These traits would also provide the complex psychological construct from which he reacts and follows rather than leads as when on the field of battle. In the end, his courage in battle shows itself as calamity reigns down upon his guilt and anger and despair.
Because we first hear of Macbeth in the wounded captain’s account of his battlefield valor, our initial impression is of a brave and capable warrior. This perspective is complicated, however, once we see Macbeth interact with the three witches. We realize that his physical courage is joined by a consuming ambition and a tendency to self-doubt—the prediction that he will be king brings him joy, but it also creates inner turmoil. These three attributes—bravery, ambition, and self-doubt—struggle for mastery of Macbeth throughout the play. Shakespeare uses Macbeth to show the terrible effects that ambition and guilt can have on a man who lacks strength of character. We may classify Macbeth as irrevocably evil, but his weak character separates him from Shakespeare’s great villains—Iago in Othello, Richard III in Richard III, Edmund in King Lear—who are all strong enough to conquer guilt and self-doubt. Macbeth, great warrior though he is, is ill equipped for the psychic consequences of crime.
Before he kills Duncan, Macbeth is plagued by worry and almost aborts the crime. It takes Lady Macbeth’s steely sense of purpose to push him into the deed. After the murder, however, her powerful personality begins to disintegrate, leaving Macbeth increasingly alone. He fluctuates between fits of fevered action, in which he plots a series of murders to secure his throne, and moments of terrible guilt (as when Banquo’s ghost appears) and absolute pessimism (after his wife’s death, when he seems to succumb to despair). These fluctuations reflect the tragic tension within Macbeth: he is at once too ambitious to allow his conscience to stop him from murdering his way to the top and too conscientious to be happy with himself as a murderer. As things fall apart for him at the end of the play, he seems almost relieved—with the English army at his gates, he can finally return to life as a warrior, and he displays a kind of reckless bravado as his enemies surround him and drag him down. In part, this stems from his fatal confidence in the witches’ prophecies, but it also seems to derive from the fact that he has returned to the arena where he has been most successful and where his internal turmoil need not affect him—namely, the battlefield. Unlike many of Shakespeare’s other tragic heroes, Macbeth never seems to contemplate suicide: “Why should I play the Roman fool,” he asks, “and die / On mine own sword?” (5.10.1–2). Instead, he goes down fighting, bringing the play full circle: it begins with Macbeth winning on the battlefield and ends with him dying in combat.