Macbeth's immoral behavior results from his overwhelming ambition for power and wealth: He wants to become King of Scotland, no matter the cost to his character or even to his immortal soul. Once he gives in to his "black and deep desires," Macbeth sets out to gain the crown, committing numerous immoral acts, one after another with increasing frequency.
Macbeth's secret desire to become King of Scotland first turns him from an honest and valiant soldier and friend into a liar and deceiver--and then into a murderer of innocent men, women, and even children. Before he is finally destroyed, Macbeth murders King Duncan, casts guilt upon Duncan's sons, arranges the murders of Banquo and his son (Fleance does escape), and orders the destruction of Macduff's entire household, including his wife, children and servants.
Macbeth's immorality and depravity are great, and even he understands the depths to which he has sunk to gain power:
Will all great Neptune's ocean wash this blood
Clean from my hand? No; this my hand will rather
The multitudinous seas incarnadine,
Making the green one red.
Macbeth understands how much blood he has on his hands, but it does not deter him. The strength of his ambition is far greater than that of his conscience.
At the beginning of the play, Macbeth struggles with the decision of whether or not to kill Duncan. In Act I, scene 7, he wrestles with his decision and produces several good reasons against carrying out the murder. He says of Duncan, "He's here in double trust: / First, as I am his kinsman and his subject, / Strong both against the deed; then, as his host" (10-12). In other words, Macbeth owes Duncan his fidelity and loyalty because he is both a subject and a relative of Duncan, and he is also his host. In addition, Macbeth says of Duncan:
Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued against
The deep damnation of his taking-off. (14-18)
In other words, Duncan has been such a fair king that his virtue will speak well of him when he dies. Macbeth feels it is unjust to kill such a fair leader. Macbeth also fears that his bloodiness in killing Duncan will spur others to act in a similarly violent fashion, which is a form of moral reasoning.
In presenting these reasons not to murder Duncan, Macbeth is wrestling with his morals. However, even after presenting the moral and rational reasons not to kill Duncan, Macbeth's moral qualms fall away when he considers his ambition. He says, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition, which o'erleaps itself / And falls on the other" (25-28). In other words, he says that only his untamed ambition to be king makes him want to kill Duncan, in the same way that people rush forward and fall on each other. Ambition pushes him towards murder and causes the breakdown of his morals, to the point where he decides to kill the rightful occupant of the throne and usurp it for himself.