Even as a loyal soldier, Macbeth was ambitious. He was clearly pushing himself. He also had no problem killing in battle, demonstrating that he was not wary of taking a life when he needed to. He was also very impressionable, making it easier for others to manipulate him. Unfortunately, these traits caused him to become a brutal killer.
The witches took advantage of Macbeth’s curiosity and bloodlust to push him in the direction of killing Duncan. After hearing the prophecies, Macbeth was sure he was going to be named Duncan’s successor. When he wasn’t, his rage and ambition became clear.
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.(Act 1, Scene 4)
Lady Macbeth knew her husband well. She felt that he was not going to take the action she thought he needed.
Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene 5)
She was right. Macbeth was wary, but Lady Macbeth made it easier for him. She told him to be a man, and feed his ambition. Since she planned out every aspect of the murder, it was easy for him to follow along.
Macbeth’s pride and arrogance is such that once he has the kingship, all he cares about is keeping it. His ambition rules his life. He kills his best friend, Banquo, because he is worried that Banquo knows too much—but also because of the prophecy that Banquo’s sons would be king. Macbeth does not only want the crown for himself, he also wants to pass on the honor.
When first they put the name of King upon me,
And bade them speak to him; then prophet-like
They hail'd him father to a line of kings:
Upon my head they placed a fruitless crown… (Act 3, Scene 1)
Macbeth’s ambition is also his downfall. He begins to act erratically, and his subjects lose faith in him. When he sends murderers to slaughter Macduff’s family, it is the excuse Malcolm needs to storm Inverness, and take back the throne. Macbeth hears a new set of prophecies that are warnings of danger, but which he interprets as harmless. As such, his ambition spells his doom.
Ambition is not in and of itself a bad thing. In fact, it is important for a soldier or king to be ambitious. Yet unchecked ambition, coupled with arrogance, is the path to self-destruction for Macbeth.