Your question assumes that Macbeth is a virtuous man when Macbethby William Shakespeare begins; however, that seems to be a pretty big assumption considering that he is moved to kill a king (a mortal sin) because of a few simple words spoken by three old women. That would not be enough for a virtuous man to commit such an evil act.
In any case, Macbeth gives no definitive evidence of wanting to be king at first, though it is not long before he reveals his hidden ambition. When Duncan announces that his son will be heir to the throne, Macbeth says:
Stars, hide your fires;Let not light see my black and deep desires:The eye wink at the hand; yet let that be,Which the eye fears, when it is done, to see.
Getting rid of Malcolm is expressed as a metaphor--a step which Macbeth must "o'erleap." He tells the stars to hide their light (personification) because he wants to hide his "black and deep desires," which can cause his eye to fear (personification).
The last line of the first act also indicates his move toward being a "murderous tyrant":
False face must hide what the false heart doth know.
This metaphor of wearing a mask to hide evil and evil intentions is a common theme in this play.
Shortly before the Murder, Macbeth starts seeing things; this is a different kind of imagery, as it is something he sees but does not physically exist:
A dagger of the mind, a false creation,Proceeding from the heat-oppressed brain.
The next imagery which depicts Macbeth's transformation into a killing machine are his bloody hands after he has returned from killing Duncan.
Will all great Neptune's ocean wash this bloodClean from my hand? No, this my hand will ratherThe multitudinous seas in incarnadine,Making the green one red.
The reference to Neptune is an allusion, and an ocean of blood is hyperbole. These are both lofty images which imply a huge guilt for, of course, one of the worst sins. Everything here is big, like his evil act. Later, he tells his wife:
O, full of scorpions is my mind, dear wife!
Again we see that Macbeth is plagued with guilt for his heinous act. At the banquet, also, Macbeth describes the guilt which is plaguing him, this time using alliteration for emphasis:
But now I am cabin'd, cribb'd, confined, bound inTo saucy doubts and fears.
The situation in Scotland is terrible, and the state of Macbeth's mind is no better. He is paranoid and quick to kill anyone who disagrees with him, or anyone he thinks might possibly be plotting against him, or just anyone he happens to think might, perhaps, be a threat to him. In the environment Macbeth has created, suspicions and fear are as rampant as his evilness.
Before he goes to see the witches again, Macbeth uses a metaphor to explain where his murderous acts...
have put him:
I am in bloodStepp'd in so far that, should I wade no more,Returning were as tedious as go o'er:
Finally, Macbeth knows his life is over; he personifies life (it creeps) and days (they have lit the way to death). The last lines use metaphors to describe his helpless life--a "brief candle," a shadow, and a poor actor who eventually just fades into obscurity and dies.
Out, out, brief candle!Life's but a walking shadow, a poor playerThat struts and frets his hour upon the stageAnd then is heard no more: it is a taleTold by an idiot, full of sound and fury,Signifying nothing.
Macbeth's life, because of his evil deeds, has become meaningless.