Macbeth is the protagonist of William Shakespeare's Macbeth, and the source of all his problems, his "fatal flaw," as you call it, is his ambition.
When the witches indicate that Macbeth will one day be king and the first two predictions are realized, Macbeth is amazed, but already he is contemplating the possibility.
[Aside] Glamis, and thane of Cawdor!
The greatest is behind.
In a moment, however, he again becomes the loyal soldier and says two things:
[Aside] If chance will have me king, why, chance may crown me,
Without my stir.
[Aside] Come what come may,
Time and the hour runs through the roughest day.
It appears that Macbeth is resigned to waiting for the day when he assumes he will become king; but his ambition is just below the surface. Unfortunately for Macbeth, in the next scene, King Duncan pronounces that his son, Malcolm, will be his heir. Now things change for Macbeth, and he says:
[Aside] The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies.
There it is, the ambition which will go on to cause Macbeth to kill the king, his best friend, and anyone else who seems to be against him (for he becomes quite suspicious and paranoid) in order to get and keep his power.
Macbeth's ambition (which causes him to kill Duncan) vaults him to the throne, but it is also the thing that kills him. Scotland is suffering at his hands because he is concerned only about maintaining his position. When he sees the witches one last time, they make him more promises, which, in his arrogance and blind ambition, he chooses to believe. If he had not assumed that no one could kill him, Macduff might not have been able to do it.
Macbeth's ambition, as well as that of his wife, is what steals the throne for him; he is unworthy to be king but remains in the position until his ambition and arrogance finally lead to his death. Ambition coupled with arrogance is Macbeth's "fatal flaw."