Before the prophecies, the witches all agree that the day is disgustingly beautiful:
Beautiful is disgustingly filthy, and disgustingly filthy is beautiful.
On this day, Macbeth has won the battle and that could make the day disgustingly beautiful. With all the blood shed, the day is no doubt disgusting yet beautiful because Macbeth has won the battle.
Right before Macbeth meets the witches, he echoes their thoughts:
I have never seen a day that is so disgustingly filthy and beautiful.
The witches meet Macbeth and prophecy amazing prophecies:
FIRST WITCH: All hail, Macbeth! Hail to you, Baron of Glamis!
SECOND WITCH: All hail, Macbeth! Hail to you, Baron of Cawdor!
THIRD WITCH: All hail, Macbeth! That shall be king hereafter!
Macbeth is beside himself with such amazing prophecies. He immediately begins to think about how these prophecies could become true. When Ross and Angus meet him and call him Baron of Cawdor, he questions how this could be when the Baron of Cawdor is alive.
When Ross and Angus share the good news, Macbeth begins to think about the witches' prophecy in that he would become king hereafter. Macbeth begins having horrible imaginings that cause his hair to stand on end. He talks to himself:
[Aside.] This supernatural meeting
Can’t be bad, only it can’t be good either. If it’s bad,
Why has it given me promise of success,
That began with a truth? I am Baron of Cawdor.
If it’s good, why do I give in to that suggestion
Whose horrid image makes my hair stand on end,
And makes my heart pound so hard they knock at my ribs,
Against my will to stay calm? My current fears
Are less than horrible imaginings.
My thought, whose murder is still only a fantastic idea,
So shakes my manhood, that functioning like a man
Is smothered in unfounded allegations; and nothing is
Only what is not.
To become king, Macbeth realizes that King Duncan must die. The witches said he would be king hereafter. This thought sends shivers up Macbeth's spine. It is not easy to to murder a king. The witches said that hereafter Macbeth would be king. He is not king yet. King Duncan must die. Images of King Duncan's murder frightens Macbeth. The reader knows that he is thinking of murdering King Duncan. Still, Macbeth is frightened at such a thought.