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:

readability="11">

...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: 

readability="8">

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.