Macbeth and Lady Macbeth go against their natures by murdering Duncan because Lady Macbeth has to rely on her husband, and Macbeth has to act on his ambition.
Macbeth and Lady Macbeth make a good team. He can’t act without her propelling him, and she can’t get anywhere without him. Without one another, they are nothing.
Lady Macbeth’s nature is to take charge and do what needs to be done. Macbeth is fully aware of this. Maybe he finds it attractive. When she pushes him, he admires her.
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (Act 1, Scene 7)
However, she has to push him pretty far in order for him to actually act. She is fully aware that he does not normally get things done by himself.
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene 5)
Lady Macbeth does not like having to rely on anyone, but she can’t be queen unless Macbeth is king. Macbeth also does not like to act. He wheedles and warbles until she finally tells him exactly what to do and makes sure he does it, step by step.
After the deed, Lady Macbeth regrets not being in control. Macbeth basically tells her to step back and let him do his thing, and not tell him what to do anymore. This really annoys her. She wants to tell him what to do, and does not really trust him.
She is wise to worry. Macbeth seems to like murder. As soon as he has power, he only thinks about losing it. He kills Banquo, Macduff’s wife and son, and countless others. He does this all to maintain his kingship. He essentially loses his grip on reality and his self-control.
It is too much for Lady Macbeth. She can’t wash the metaphorical blood from her hands.
The Thane of Fife had a wife; where is she
now? What, will these hands ne'er be clean? No more o’
that, my lord, no more o’ that. You mar all with this(40)
starting. (Act 5, Scene 1)
It is not the murder of Duncan alone that bothers her. It is her inability to stop or control Macbeth. She kills herself.
Ultimiately, Macbeth can't fight his real nature either. He is a coward at heart. He keeps killing until he is finally killed, thinking of nothing but losing what he has. As the play demonstrates, when we act against our natures it is destructive, and devestating.