The metaphor, of course, refers to horses and horsemanship. Hissoliloquy is interrupted by the entrance of Lady Macbeth. He intended to say, "...and falls on th' other side." This metaphor suggests an inexperienced and rather ridiculous rider who tries to vault onto a horse and vaults so vigorously that he goes right over the saddle and falls in a heap on the ground. As far as this relates to Macbeth's ambition, he is foreseeing that it will be a serious mistake to murder Duncan because he has no excuse for doing so and because his misdeed will lead to his own ruin.

When Lady Macbeth interrupts his soliloquy, he tells her the conclusion he has come to in the following words:

We will proceed no further in this business.
He hath honored me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.

He is not telling her all his reasons but only what he hopes will appel to her pragmatic nature. Notice how he starts off so decisively and ends with what would seem to be a question deserving a question mark:

Not cast aside so soon?

Once again Lady Macbeth has to use her formidable powers of persuasion to rekindle her husband's ambition and resolution. She is the stronger of the two. He doesn't want to murder the King. We can hear the strong feeling of love and reverence in his voice when he asks his wife, "Hath he asked for me?" He not only loves Duncan, but he knows that Duncan loves him.

Whether or not his wife really loves him is problematical, but she has a strong hold over him. She is not only more ambitious and more ruthless but more intelligent. Macbeth is dependent upon her. Such marital relationships are not uncommon. When two people get married, one or the other will usually take the leadership and make all the major decisions for both. This can be observed in almost every marriage. Lady Macbeth "wears the pants in the family." Shakespeare waanted to make Macbeth look like a tragic hero by pinning much of the blame for Duncan's murder on the three witches, on "fate," and on Lady Macbeth. Shakespeare wanted to create a good man who performs a wicked deed, a hero who commits a heinous crime. But he makes Macbeth look weak, indecisive, feckless, uxorious, and like an athlete who is all brawn and no brains. Whenever Macbeth has to make decisions on his own, he makes a mess of things. One of his worst mistakes was to order the murder of Macduff's wife, children, and everyone else in his castle.

This is one of the most revealing and damning of all staements in the whole of the play 'Macbeth' by William Shakespeare. It is a quotation which does indeed refer to the statement

"To what extent is Macbeth responsible for his own downfall?"

because for hundreds of years experts and critics have been arguing about the extent of MacBeth's culpability for the murder and for his own tailspin down into potential madness. In fact, the jury is still out, and that is why we have to look so carefully at the playwright's own language and to scrutinise and analysis the dialogue in context, so that we may form an opinion. Every reader and playgoer will have a valid response and your opinion is just as valid as anybody else's - porvided that you study the play - because we are all coming at it from different angles. You bring a different set of life experiences to the text to another person.

The lines

'I have no spur, To prick the sides of my intent, but only Vaulting ambition, which o'erleaps itself And falls on the other. ' (1.7.1)

suggest owning the decision, turning a back on counsel, advice and conscience and in Elizabethan times turning one's face away from God deliberately was seen as the deepest most evil offence against the Holy Spirit. The decision is like a horse, needing no spurring onwards, having a mind of it's own. The evil path is intentional, riding on personal gain and ambitious plans.

One complication however, is that those suffering psychosis, can often act in a coldly rational way and seem to all others and colleagues around them, perfectly lucid and sensible until the last minute!

At the beginning of Act One, Scene 7, Macbeth candidly speaks about murdering King Duncan and contemplates the potential consequences of his actions. Macbeth states that if killing Duncan would be the only crime he would need to commit, then he would do it without hesitating. However, Macbeth is aware that assassinating King Duncan will only result in more bloodshed. Macbeth then lists the several reasons why he should not murder the king. Macbeth knows that the king trusts him because Duncan is his relative, and Macbeth also believes that it is wrong for a host to harm his guests. Also, King Duncan is an honorable man and a gracious leader, who does not deserve to be assassinated. After listing the numerous reasons to not follow through with the assassination, Macbeth says,

"I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself / And falls on th' other" (Shakespeare, 1.7.25-28).

Essentially, Macbeth is admitting that his ambitious nature is the only thing motivating him to assassinate the king, which is something that leads men directly toward disaster. Macbeth acknowledges that ambition is his only driving force and understands that there will be consequences for his actions.

At the beginning of act 1, scene 7, Macbeth considers the myriad compelling reasons he has not to kill the king—Duncan is his friend, his king, his relative, his guest.  Further, it is not as though Macbeth will simply commit the murder and then become king; there are more steps that will have to be taken. Not to mention the facts that committing this act would corrupt Macbeth's immortal soul and that Duncan has been a good, virtuous leader.  Moreover, if he murders the king, then Macbeth shows other people that one can, in fact, murder a king. Because "Bloody instructions, which, being taught, return / To plague th' inventor," someone may use that knowledge to murder Macbeth in the end (1.7.9-1). After Macbeth lists all his reasons not to kill Duncan, he says,

I have no spur 
To prick the side of my intent, but only 
Vaulting ambition, which o'erleaps itself
And falls on th' other  (1.7.25-28).

In other words, Macbeth has no impetus, no reason to move forward with the murder except for his great ambition, a quality which has a tendency to make people commit rash actions that lead to their downfall. He has many reasons not to kill Duncan and only one reason to do it: ambition.