When he first enters the stage, Macbeth remarks, "So foul and fair a day I have not seen" (1.3.39) words which underscore the paradoxical motif of the play introduced by the three witches in the opening scene. And, the act of killing King Duncan is both "foul" and "fair" for Macbeth. For, it is most "foul" to commit regicide, especially killing a man who has been a friend; however, regarding Macbeth's "vaulting ambition," it is a "fair" act since that act of murder advances him as he is made king after Duncan's death.
Subsequent to Macbeth's initial remark, he encounters the witches and is hailed as "King hereafter." After this salutation, Macbeth confers with Banquo and in an aside he ponders what has occurred, surmising that "nothing is/But what is not" (1.3. 152-153), considering that reality and fantasy are equal. Thus, the killing of King Duncan represents Macbeth's lust for power and his distorted sense of reality with fantasy since he rationalizes that his regicide is merely a chance fulfillment of the prophesy of the witches:
If chance will have me King, why, chance may crown me. (1.3.155)
And with his vision of the dagger and imaginings before his act of murder, the killing of King Duncan further illustrates the theme of Appearances vs. Reality.