I personally agree with Thomas De Quincey's theory on The Knocking of the Gate before Duncan's murder in Macbeth. Quincey had stated that a murderer in Shakespeare's usual plays must have, "...some great storm of passion--jealousy, ambition, vengeance, hatred--which will create a hell within him, and into this hell we are to look." Ambition is what Lady Macbeth and the witches planted in Macbeth. It was his ambition to become king after he considered that it had been his "destiny" to recieve the crown in any way possible. The witches however, didn't state what striving to become king would do to him and how the murder of Duncan would affect him personally at least not until it was already too late. As Quincey stated, "...the knocking at the
