Macbeth is traditionally considered to be a tragic hero and tragic heroes always have a fatal flaw of which Macbeth's is undisputedly his unchecked ambition.

Of the many problems that Macbeth brings upon himself, it is his ambition that drives him. He is aware of this and that his "Vaulting ambition...o'erleaps itself..."This is the thing that will ultimately destroy him. He allows his ambition to discount his better judgment.

Even when the odds are stacked against him such is his need to be king that he " will not yield, To kiss the ground before young Malcolm's feet.."

He is intrigued by the witches and does trust them, to the point of visiting them to be sure of his path but he not trust them sufficiently to  leave the timing of his rise to kingship in their hands.He does, however, contemplate it

if chance will have me king,why chance may crown me/ without my stir..

but with his own ambition and Lady Macbeth's consuming ambition, leaving Macbeth's future in the witches hands is not enough. He has even had his friend killed (Banquo) just to be sure - in case Banquo should cast suspicion on Macbeth.

He trusts Lady Macbeth and without her he would faulter. She even shocks her audience with a graphic visual image when she suggests that she would harm a child and  

pluck'd my nipple from his boneless gums,
And dash'd the brains out.

and would do anything to ensure his success but, once he realizes what he can do on his own, he no longer needs her.

In Macbeth, there can be no doubt that

ambition conspires with unholy forces to commit evil deeds.

What may be acceptable on the battlefield is not so in normal society and Macbeth became unable to distinguish between the two. Having been praised and rewarded for exploits in battle and protection (ironically) of the king and crown, he transferred his prowess and applied it to further himself personally.

This is a failing of man and is indicative of the "human condition," especially in view of ongoing wars that most countries find themselves involved in.  

Abuse of power is a sad and easily recognizable trait which can be seen the world over. Leaders get so carried away that they fail to accept responsibility for their own involvement in the breakdown of society. Under the guise of leadership, their ambition takes over and any good intentions often get sidelined. Some leaders are so worried that if they do not get personal gain someone else will just come along anyway. Some feel that it is their right and they "need" to establish a certain status.

Again this stems from man's own moral breakdown. Have we really forgotten what it's all about?