Why does Macbeth have second thoughts about killing Duncan?
An important scene, Scene 7 of Act I of Shakespeare's Macbeth presents what is the tragic flaw in a man who has the potential to be noble if he would but listen to his conscience. For, in this scene, Macbeth struggles with his conscience in an internal conflict between his "vaulting ambition" and his blood ties and loyalty as a Scottish nobleman to his king, Duncan as well as his fear of eternal damnation.
In his soliloquy while preparations are being made in his castle for the honored dinner guest, King Duncan, Macbeth considers the argument against his murdering his king:
First, as I am his kinsman and his subject
....then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself.
Then, Macbeth reflects that Duncan, besides being his relative and king, is a virtuous man:
...Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels...
And, as such a virtuous man, if Duncan is killed, surely the murderer risks damnation:
The deep damnation of his taking-off....
Realizing that he has no reason, no just motive for killing Duncan, Macbeth recognizes that it is only his desire for power that overrides all the other reasons:
Valuting ambition, which o'erleaps itself
And falls on th' other (1.7.13-28)
Thus, Macbeth's concession to his cupidity over his conscience is his tragic mistake, for it effects his later demise.