Who is responsible for Macbeth's demise in Macbeth?

Although many factors contribute to Macbeth's imminent downfall, the play suggests that Macbeth alone is to blame for his own demise. He admits that his own ambition is the very reason he cannot prevent himself from committing evil deeds:

Stars, hide your fires;Let not light see my black and deep desires...

I have no spurTo prick the sides of my intent, but onlyVaulting ambition, which o'erleaps itselfAnd falls on the other.

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Although many factors contribute to Macbeth's imminent downfall, the play suggests that Macbeth alone is to blame for his own demise. He admits that his own ambition is the very reason he cannot prevent himself from committing evil deeds:

Stars, hide your fires;
Let not light see my black and deep desires...

I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other.

We know that Macbeth is encouraged by his wife and the witches' prophecy to pursue his ambition. He needs to murder king Duncan if he wants to ascend the throne. Nevertheless, his inability to stifle his unrestrained ambition causes him to transform himself from a loyal and valiant warrior to an evil tyrant. Once Macbeth kills the benevolent king Duncan, he does not stop until he gets rid of anyone who he feels threatened by (Banquo, Macduff's family, etc.).

By embracing evil, Macbeth deprives himself of peace and happiness. In Act V, he realizes that his life has become utterly meaningless: 

It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.

Macbeth has free will, and he chooses to follow through with his evil plan. Therefore, he alone is responsible for his downfall.