In Shakespeare's Macbeth, Macbeth is beguiled by the witches' predictions. In Act One, scene one, the witches are waiting for Macbeth—they have targeted him. When Macbeth and Banquo arrive from battle, the witches greet Macbeth by name—then by the title of Cawdor that belongs to another, and finally as king:
All hail, Macbeth, that shalt be King hereafter! (I.iii.53)
Macbeth is intrigued by what they say, but it is not until he is greeted with the news that he will been given the title and lands of the traitor Thane of Cawdor that his interest is truly piqued. Even Banquo (who heard the predictions) is amazed, for the Elizabethans (of which Shakespeare was one) believed that the devil was unable to speak the truth:
What, can the devil speak true? (113)
With the promise of the witches' words ringing in his brain, Macbeth begins to consider the possibility that he might become king. Since Duncan is healthy and has two sons, the road to such an event is not a clear one: Duncan's death, ultimately, must take place. It occurs to Macbeth that it might take place without him lifting a finger, and this reasoning might have stayed his hand...but then he returns home to discuss it with his equally ambitious—and treacherous—wife.
When they meet, Lady Macbeth makes it clear to her husband that even though Duncan—who will stay with them overnight—intends to leave the next day, he will never live to see that dawn—for she has plans to murder him.
At the start of scene seven, Macbeth admits in a soliloquy that he has one flaw that foreshadows his potential for evil—he is very ambitious:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other— (I.vii.25-28)
Then Macbeth tells his wife that he is having second thoughts:
We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (I.vii.34-38)
Furious, Lady Macbeth berates him, calling him a drunk, a coward and a liar—that he has made promises to her that he now wants to break. Her personal desire to become queen brings out a truly evil side of her character as she describes what she is capable of doing...
I have given suck, and know
How tender ’tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out, had I so sworn as you
Have done to this. (I.vii.60-65)
Macbeth is taken aback by his wife's words. In the face of such cruelty, he tells her to only have male children, for she has no gentleness in her.
Macbeth is a hero; murder does not come easily to him, but he gives in. After he kills Duncan, he returns to his room with the bloody daggers, and is so shaken that he refuses to take them back. However, he notes that with practice it may become easier. This is proven later than morning when he becomes so "upset" by the King's death that he kills the guards (potential "witnesses").
The witches' second set of predictions give Macbeth a false sense of security. Hecates says:
Is mortals’ chiefest enemy. (III.v.32-33)
Once he kills Duncan and forfeits his soul (for regicide), more killing means little. In his dark ambition, Macbeth arranges for the murder of his friend Banquo, and later Macduff's family. He is so steeped in evil, that nothing can undo what he has done.
The actions of Macbeth that make him truly evil is actually very straightforward. He opens his house to Duncan with all hospitality. In the time of the Elizabethans, hospitality was directly linked to honor. When a guest was welcomed into a home they were not only provided food, drink and comfort. They were also assured complete safety. The safety was extended even to mortal enemies. If hospitality was offered, then the guests safety was paramount. In killing his guest, Macbeth took the final step into complete evil.