What causes Macbeth to kill the king?

As he begins to doubt himself, and his decision to murder Duncan, Macbeth wonders what his motivation is:

‘I have no spur
To prick the sides of my intent but only
Vaulting ambition which o’erleaps itself
And falls on th’other’
— Act I, scene 7

Because he is using language taken from horse riding to make his point (spur, prick the sides) the word vaulting makes sense. Horses jump over obstacles and Macbeth’s ambition will propel him to clear the obstacle of Duncan. However, the word also has other suggestions, which Macbeth realises. Ambition is something of a problem – it may help the rider clear an obstacle, but it may also make the rider go down. Macbeth cannot finish his thought, as he is interrupted (he would probably have said ‘side’). So the image which hangs in the air is of ambition, either rising or falling or both.

Although Macbeth recognises the motivation given to him by his ambition to be king, the worry of falling still gives him doubts. This has been predicted already, by his wife. Having received the letter which describes the encounter with the Witches, Lady Macbeth fears her husband’s character is not murderous enough to take the opportunity. ‘Thou would’st be great’, she says, meaning that Macbeth desires the throne:

‘Art not without ambition, but without
The illness should attend it’
— Act I, scene 5

So for Lady Macbeth, ambition is not enough. ‘Illness’ – that is, wickedness – must accompany it, if the ambition is to be realised. It is this ‘illness’ which Macbeth finds himself lacking in as he prepares to murder Duncan.

This leads us to wonder: what causes Macbeth to kill the king? His ambition comes only from the prophecy of the Witches. He does not dream of the throne until he is told that such a dream will come true. But when that ambition is set, he is still unsure whether to murder:

‘If chance will have me king, why chance may crown me
Without my stir’
— Act I, scene 3

Macbeth realises he does not have to do anything to take the throne. So the murderousness does not come straight from the promises of the Witches, but from somewhere else.