Is Macbeth's fatal flaw ambition or pride?
Macbeth's fatal flaw seems to be ambition. After the three witches reveal the prophecy that Macbeth will one day be king, Macbeth and his wife hatch a plan to make sure that happens. The idea of murdering someone never entered Macbeth's mind until he heard about the possibility of being king.
In Act 1, Scene 7, Macbeth even admits that the only reason he has to kill King Duncan is ambition. He says, "I have no spur to to prick the side of my intent, but only vaulting ambition, which o'erleaps itself and falls on th' other."
Even after Macbeth kills King Duncan and becomes King of Scotland, his ambitions continue. He hires murderers to kill Banquo and his son Fleance because he wants to make sure his dynasty continues to his future son.
All of the murderous decisions Macbeth makes in the play are a result of him wanting to be king, and then once he receives the crown, making sure he stays king and that it continues within his lineage.