What is the nature of temptation in the play Macbeth? What tempts him, and how does he handle this temptation?

Macbeth's tragic flaw is his ambition. What tempts him is the desire for power, which is whetted by the three witches prophesying that he will become King of Scotland.

The scenario unfolds as follows: Macbeth helps win a battle in support of King Duncan, and unbeknownst to him, the King declares him Thane of Cawdor as his reward. Macbeth, with Banquo, then encounters the witches on a wild heath and they address Macbeth as Thane of Cawdor. They also say he "shalt be king hereafter."

Macbeth is startled and questions the witches, but they disappear.

When Macbeth finds out he is indeed Thane of Cawdor, this causes both him and Banquo to wonder whether the witches are also right that he will become king. Banquo warns Macbeth that the witches, who he calls "the instruments of darkness," may have told him one truth (about becoming Thane of Cawdor) to "betray" him and lead him down a dark path to his own destruction.  

The problem is that the words of the witches speak to Macbeth's deepest desire: to become king is the thing he wants. He wants to believe their words are true, and he and his wife yearn for him to be king so badly that they are not willing to wait for events to unfold naturally: they both believe they need to make it happen right away, through murder, because Duncan is staying with them and they don't know when the opportunity to kill him will arise again. Macbeth, even before consulting Lady Macbeth, is tempted with the "horrid image" of murdering Duncan, an image that "doth unfix [his]...hair [makes his hair stand on end]."

Macbeth handles the temptation badly by giving into it and murdering Duncan, his king, to whom he has pledged his loyalty. This is a deep betrayal. He is goaded on by his wife and the witches, but he himself is to blame for he gives in to the dark impulses inside himself: though he has a moment of struggle in which he thinks that once he steps over the line into murder, it will be all bloodshed for the rest of his life and no going back, he still does what he knows is wrong.