Prior to becoming king, even prior to his first encounter with the witches, Macbeth is a Scottish nobleman, a general in the king's (Duncan's) army. The Sergeant reports to Duncan and Malcolm that Macbeth has fought bravely in battle:
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,
Like valor's minion carved out this passage
Till he faced the slave,
Which ne'er shook hands nor bade farewell to him,
Till he unseam'd him from the nave to the chaps,
And fixed his head upon our battlements. (I.ii.18-25)
Upon hearing this and then the Thane of Cawdor's dismal performance in battle, Duncan declares that Macbeth shall be given the title, Thane of Cawdor. Macbeth, in the next scene, hears this as a prophecy from the witches. When it is confirmed by Ross that Duncan is to give Macbeth this new title, Macbeth really starts to consider the prospect of becoming king:
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme!—I thank you, gentlemen. (I.iii.138-40)
At this point, Macbeth begins to consider how he would become king: the seed of greed is sown. He sees Duncan and Malcolm, the rightful heir, as obstacles. Macbeth begins to reluctantly consider murder as a means of overcoming these obstacles. However, at nearly every step of his plans to become king, Macbeth is consumed with anxiety and/of guilt.
In Act Two, Scene Four, following Duncan's murder, the general atmosphere in Scotland is strange and disconcerting. By this point, Malcolm and Donalbain have left Scotland, arousing suspicion and Macbeth has been named successor to the king. The further Macbeth goes along, the more anxious he becomes. As such, he becomes completely paranoid; he will do anything to get rid of any threats to his power. After having Banquo killed, Macbeth hallucinates, seeing Banquo at a banquet. In the next act, when Macbeth discovers that Macduff could also be a threat, he makes arrangements to deal with Macduff and his family. As one lie often leads to more lies, Macbeth's sins lead to more sins. His crimes lead to paranoia which leads to more crime. In the end, he has transitioned from a loyal nobleman to a paranoid and conflicted murderer.