Why couldn't Macbeth withstand his ambition to do wrong.
This seems like a simple question, but this is the crux of the entire play. What you need to decide is whether or not Macbeth determines his own actions or is being manipulated by others.
From the description of Macbeth and his actions on the battlefield in Act I, scene ii he appears to be a loyal, capable thane. Putting down a rebellion against Duncan seems to demonstrate a lack of murderous ambition. We get this first impression of Macbeth second-hand, though--we'll never really know what the man was like before the play begins.
The witches--from the first moments of the play--appear to have a hand in events. How much? That is for the audience to decide. When they speak their first prophecies to Macbeth in Act I, scene iii, Macbeth's mind immediately turns to murder: "why do I yield to that suggestion / Whose horrid image doth unfix my hair / And make my seated heart knock at my ribs / Against the use of nature?" Either Macbeth's ambition was always there, just waiting for an excuse to escape, or the witches are manipulating him--perhaps even casting some kind of spell over him.
Your answer to this question of Macbeth's culpability is the key to your understanding of the play. Either he was a good man who turns murderous in an instant or he is the victim of a supernatural conspiracy. Your view will color any analysis of theme--particularly how tragic a figure Macbeth really is.
Macbeth could not withstand his ambition for doing wrong because of his lack of brutality and his inert humanity.
William Shakespeare's Macbeth depicts Macbeth as a man who has a hidden ambition to be the king, but this is just like dream to him. He never thinks to execute his plans until the "weird sisters" foretell him that he, once, would be the king, and his wife poured oil into the fire of ambition. He seems to have desire, but no wicked motive to achieve his goal. And, this is clearly expressed through Lady Macbeth's speech: "yet do I fear thy nature, / It is too full o'th' milk of kindness/ To catch the nearest way.../ What thou wouldst highly, /Thou wouldst holily" (Act 1, Scene 5).
Later, once prvoked and instigated, Macbeth is deeply drowned into the bloody sea of crime. He can not be called a coward for not withstanding his ambition to do wrong, rather it is his sense of morality which resists him from doing so. His ambition needed a spur, and after getting it, he begins to lose that sense.