The fact that we can even consider this question is an answer in itself. Nothing is ever simple in Shakespeare!
In Act I, scene v Lady Macbeth introduces us to the idea that masculinity is the key to success. She worries that her husband--who is coming back from a day of successfully killing hordes of enemy soldiers, by the way--"is too full o' th' milk of human kindness / To catch the nearest way." Soon after she asks spirits to "unsex" her so that she will be capable of carrying out the murderous plot. Lady Macbeth clearly feels that murder is a man's job.
Lady Macbeth continually questions her husband's masculity in the first part of the play--even to the point the Macbeth exclaims "Prithee, peace! / I dare do all that may become a man; / Who dares do more is none" (Act I, sc. vii). Macbeth, too, is struggling with equating manliness with the ability to commit cold-blooded murder.
But this is the problem, isn't it? What is the "manliness" your question refers to? If it is Lady Macbeth's then you may apply it to her when she conceives of the plan and Macbeth when he carries out Duncan's murder (and subsequent murders); to them, manliness is murderousness.
If the question asks for a more rational interpretation of "manliness," then we of course have much different ideas. Duncan's murder is the height of cowardice and dishonor--far from the acts of a true man, right?
Define manliness first. Then answer your question in the way that seems appropriate. Many times by changing the way you approach a question or word meaning you end up with a far better analysis. Is your question truly about masculinity or irrationality. After all, how much sense does Lady Macbeth's notion of manliness make?
In the play "Macbeth" by William Shakespeare, the author shows us some of the qualities necessary to be a murderer - and not all those qualities can be considered wholly masculine. For example, the perfect untraceable murder requires cunning and forward-planning - both qualities displayed more by Lady Macbeth. Conversely, in terms of impulsive quick action (another quality often needed) it is Macbeth who displays sudden acts. yet Lady Macbeth thinks herself too feminine to carry out the murders as well as she would like, and therefore wishes to be more masculine - to have her blood made thicker to stop gentle feelings and "the milk of human kindness" reaching her heart and brain. In his lack of confidence and self-doubt, Macbeth may feel unmanly - especially in those ancient times of heroic male fighting roles.
This depends very much on your definition of ‘manliness’. Lady Macbeth is able to have the strength and clear –headedness to conceive of the cruel plot to kill King Duncan, and she is firm and persuasive in convincing her husband of the need to carry out the plan. She calls upon the powers of darkness to-
unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty!
so asking to be ‘unsexed’ rather than made masculine. However, she is unable to sustain the strength shown in planning the murder when it comes to the execution and concealment of the crime.
Macbeth wrestles with his conscience and remains troubled by the repercussions of his actions until his tragic end. His way of dealing with his torment is to continue the killing spree – perhaps a more ‘masculine’ response than Lady Macbeth’s suicide.
I am in blood stepped in so far that should I wade no more,
Returning were as tedious as go o’er.