Act I, scene vii begins with Macbeth's soliloquy in which he famously declares that, while being Duncan's kinsman, subject, and host, he ought to close the castle door upon Duncan's murderer, not be his murderer.
But here, upon this bank and shoal of time,
We'ld jump the life to come.
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself.
Lady Macbeth enters and interrupts his soliloquy by asking why he has left the banquet chamber as Duncan is asking for him. Macbeth has his last and most shinning moment when he declares that they shall go no further with their murderous scheme: he has talked himself out of murdering Duncan through the course of his soliloquy.
... his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off;
And pity, like a naked new-born babe,
Striding the blast, or heaven's cherubim, horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind. I have no spur
To prick the sides of my intent,
In response, Lady Macbeth has one of her most mean-spirited and irrational moments. She goads Macbeth on declaring that her love for him will be molded and defined by what he decides in this very moment. She declares, against all logical thought, that failing to pursue his wrongful and murderous desire--inspired by herself and embedded by the Witches--is equivalent to being a coward who will be unable to live thenceforth will valor:
... Art thou afeard
To be the same in thine own act and valour
As thou art in desire? Wouldst thou have that
Which thou esteem'st the ornament of life,
And live a coward in thine own esteem,
Letting 'I dare not' wait upon 'I would,'
Unhappily, Lady Macbeth launches into a frenzied verbal attack against Macbeth leveling at him her threats of changed and withheld love; her damming of her offspring; her accusations of cowardice and promise breaking; her accusation of his being an undone man (bear in mind, she is illogically and irrationally equating choosing not to commit murder and regicide with cowardice and unmanliness). It is her continued attack that eventually sways Macbeth to the inevitability of embracing the dark deed being thrust upon him. The scene ends with his concession and bitter-hearted agreement to continue with the murder.
I am settled, and bend up
Each corporal agent to this terrible feat.
Away, and mock the time with fairest show:
False face must hide what the false heart doth know.