Shakespeare wanted to make Macbeth a tragedy, which meant that the audience needed to have some sympathy for the protagonist. I believe that Shakespeare was trying to shift part of the blame for Macbeth's actions onto his wife, and also that he was trying to shift some part onto the Weird Sisters. Macbeth seems to be trying to talk himself out of killing Duncan.
He's here in double trust:
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. Besides, this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that 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 cherubin 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, but only
Vaulting ambition, which o'erleaps itself
And falls on the other— (I.7)
If Macbeth were simply a villain, the audience would have no sympathy for him. In that case they wouldn't feel any pity for him when he can't sleep, when he is tortured by guilty memories, and when he meets his end at the hands of Macduff. In this case. however, we do feel a tiny bit of sympathy for Macbeth throughout the play. It is hard to explain why. It must be--not because he is a nice guy--but because he feels emotions which make him seem human, even decent. He feels reluctant to kill Duncan. He feels terrible guilt right after he accomplishes the deed. He goes through torture when he has to escort Macduff to the chamber where he knows Duncan is lying dead. He tells himself that he wished he had died himself. He never gets over his feelings of guilt, grief, remorse, regret, sorrow, pity, and extreme depression. He wouldn't do it if he had it to do all over again--but it is too late.
What's done cannot be undone.
Nevertheless, he is the guilty party. He is the one who will chiefly benefit. He is the one who is an expert in killing people. He had no trouble killing an old man who was asleep in his bed. In addition to blaming him for the murder, we blame him for being such a weakling that he allows his wife to force him to do something that is clearly against his will. Using Lady Macbeth to dilute her husband's responsibility only adds shame to his guilt. He is afraid of his wife. Shakespeare invented a memorable character when he created her! But it is noteworthy that she seems to fade out of the picture after she has succeeded in persuading her husband to kill the king. Shakespeare hardly knew what to do with her afterwards, since her main function in the play was to push her husband into committing the murder and thereby, hopefully, relieve him of some part of his guilt.