What causes Macbeth's downfall in Macbeth?

Macbeth’s downfall came from allowing others to influence him and his paranoia.

Macbeth was too easily influenced by others.  The witches lead him around like a dog on a leash, and his wife pushes him over the edge.  At the beginning of the play, Macbeth is a war hero.  By the end, he is a maniacal tyrant whose paranoia leads him to make enemies left and right.

Macbeth was influenced by the witches from the very beginning.  They knew what buttons to push.  Using his ambition and pliancy against him, they fed him prophecies that he would be great someday soon.

SECOND WITCH: All hail, Macbeth! hail to thee,

Thane of Cawdor!

THIRD WITCH: All hail, Macbeth, that shalt be

King hereafter! (Act 1, Scene 3)

The witches fill Macbeth’s head with delusions of grandeur, and convince him that it is his right to be king.  Using the promotion he was just about to get was a subtle but effective trick.  It made him believe that if he got the first promotion, he would get the second.  He became Thane of Cawdor, so of course he would be king.  When Duncan chose his son as his heir instead, Macbeth’s ire was up.  He was angry at not being chosen for a rank he never deserved.

Macbeth wrote to Lady Macbeth and told her of his new fortunes, or supposed fortunes.  She latched onto the chance for Macbeth to be king and wouldn’t let it go.  Macbeth returned home not sure that he really did want to challenge Duncan, and Lady Macbeth convinced him that if he didn’t he wasn’t being a man.

We fail? But screw your courage to the sticking-place,

And we'll not fail. When Duncan is asleep—

… —his two chamberlains

Will I with wine and wassail so convince,

That memory, the warder of the brain,

Shall be a fume and the receipt of reason A limbec. only … (Act 1, Scene 7)

She is so convincing that he goes along with it, and kills the king.  She has every detail planned out, even scolding him for not leaving the knife behind when he stabs Duncan.  Macbeth is stuck.  Once he kills Duncan, he has gone down a road he can’t come back from.  He wasn’t sure whether he should do it, since Duncan had done nothing wrong.  His wife gives him no choice.

Macbeth’s doubts crystalize into paranoia.  He becomes concerned that his friend and fellow noble Banquo knows too much.  He has Banquo killed and tries to kill his son too, because the witches prophesized that Banquo’s heirs would be king.  He also has Macduff’s family killed, worried that he is suspicious.  Macduff escapes, and has become an even greater enemy to Macbeth.

Macbeth was ambitious; there is no doubt about that.  However, he was also very easy to persuade.  The witches used their influence over him to destroy him.  By telling him that no man born of woman could kill him, they made him susceptible to Macduff’s influence.  Told to beware Macduff, Macbeth did not know what to believe, and Macduff was able to manipulate him into basically giving up the fight.