Macbeth’s tragic flaw is ambition.  He wants to be king, and even though he knows it isn’t right, he allows himself to be persuaded.  At first, he puts the thought to the side, even though the witches tell him it will happen and he is named Thane of Cawdor.  He even argues with himself:

“He’s here in double trust: First, as I am his kinsman and his subject, Strong both against the deed; then, as his host, Who should against his murderer shut the door, Not bear the knife myself.”

He is obsessed with the idea of keeping the power he has gained.  His unrelenting ambition drives him to murder his friend, Banquo, and Duncan’s family. 

Lady Macbeth plays an important part in bringing Macbeth’s ambition to the forefront.  When Macbeth returns home, she tries immediately to discuss with him the idea of being king.  Because Lady Macbeth knows Macbeth so well, she appeals not only to his desire for power, but also to his male pride.  She accuses him of being a coward: “like the poor cat i’th’adage.”  Later in the play, Lady Macbeth will struggle desperately against her guilt, but at the beginning, she acts as the fuel to feed Macbeth’s desire to become king.