Macbeth indeed murders Duncan, but Lady Macbeth is equally responsible. Macbeth considers that he will be doing away with a good and beloved king, and his cousin, to boot, but Lady Macbeth's urging for him to commit the murder and thus assume the kingship is more than his own weak morals can withstand.
Consider Act 1, Scene 7, in which Macbeth is vacillating:
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 calls his a coward and taunts:
Was the hope drunk
Wherein you dress'd yourself? hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love. 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,'
Like the poor cat i' the adage?