In general, in Shakespeare's Macbeth, Macbeth's tragic flaw is his vaulting (or overpowering) ambition for power.
Aristotle defined a tragic hero as a great (accomplished) man, who has to die, and dies because of his own fault—because of a tragic flaw. A "tragic flaw" is defined as:
...the character defect that causes the downfall of the protagonist of a tragedy
It is also known as "hamartia," which is defined as:
...missing the mark, failure, fault, or error
When he plans to kill Duncan, his King, friend, relative and houseguest, Macbeth admits to the one thing that drives him forward:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition... (I.vii.25-27)
Macbeth's desire for power is the only reason he kills Duncan; it is this flaw that causes him to turn his back on the decent man he is at the beginning of the plan (a valiant warrior, loyal subject and friend) to a murderous tyrant who kills not only Duncan, but his best friend and his enemy's wife and children, among others.
Hecate, the queen of the witches, also notes another problem that haunts Macbeth: he believes he cannot be defeated because of the witches' first set of predictions seemingly came true, proving (in his mind) their credibility. Because he feels so safe, Hecate knows that Macbeth will behave foolishly, much to her delight:
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.
And you all know security
Is mortals’ chiefest enemy. (III.v.30-34)
The other flaw in Macbeth's character, found in Act Five, scene four, is based on these two elements. Macbeth is certain that he is protected because he believes he can only be defeated under impossible circumstances. He is so ambitious, that he will not heed news from his lookouts that the woods are moving.
Duncan's son, Malcolm, has given his men orders to camouflage themselves with branches to cover their numbers so Macbeth does not know how many men are in Malcolm's army.
Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (V.iv.6-9)
Because Macbeth is so certain that he is safe, he doesn't consider that if one of the witches' predictions was based on a half-truth, all of their predictions were suspect. And so, he goes into battle, still believing that Macduff cannot hurt him. Macbeth fails to consider that the witches, inherently evil, might actually be working toward his destruction. It is this "false sense of security," and his vaulting ambition that bring about his demise.