What is the character flaw that is the cause of Macbeth's downfall?

Shakespeare's plays are usually put into one of several categories: Tragedy (such as "Macbeth"), Comedy (such as "All's Well That Ends Well"), History (Such as "Richard III'), Pastoral (such as "As You Like It"), or Romance (such as "The Tempest").

In each of these genres, Shakespeare observes and comments on the human condition. In essence, nothing much has changed about human beings AS human beings from the Elizabethan era, during which he lived and worked, to our own.

The category, "Tragedy", contains Shakespeare's "Macbeth", "King Lear", "Antony and Cleopatra", et al.  In each of these, the true "tragedy" can be said to be a human flaw which sets into motion and/or keeps in motion the events which result in disaster. These character flaws have existed since time immemorial, exist in each of us today, and will likely continue to exist as long as the human race endures.

Human flaws might also be called tragic flaws, character flaws, human inadequacies, or a host of other names. By examining ourselves and those around us, we will surely observe such factors in our lives as greed, hubris, dishonesty, untrustworthiness, etc.  We can also readily understand that such flaws within us contribute mightily to the state of war, the economic state, the moral state, and the state of humanity which plague us in our own time and place.

With the above as introduction and background, one can understand that Macbeth's greatest human frailty, his vaulted flaw, the tragic fiber within him which drove him to his horrible deeds, was greatly excessive ambition.

Ambition can be a desirable characteristic in any of us.  Without ambition, we would remain complacent in such mundane activities as cleaning house, bathing, preparing healthful meals, etc.  A lack of ambition would have an even greater effect on our going beyond the most basic requirements in our studies, or being willing to remain the person lowest in rank and most poorly paid in our careers.  At the highest levels, no ambition would lead us not to have sufficient desire and drive to work for peace, or strive to cure disease, or to show compassion to our fellow man.

But ambition can also be a dangerous thing.  It was ambition which caused the break in at the Watergate Hotel and brought an American president to the shame of resignation.  It was religious ambition which brought about the horrifying events of "9/11". Political and military ambition has most often been the defining factor in bringing nations into war.

It was this same kind of "ambition on steroids", one might say, which caused Macbeth to hasten events which would have likely happened anyway, had he had the patience to await them. But, without patience, and with an ambition that was clearly out of control, he brought about horrendous evil, including the most dreadful crime of murder.

Shakespeare understood the human condition, in my opinion, better than any other writer of his or any other time.  His studies and observations in the same have resulted in plays of great power and insight.