Malcolm defines the kind of man he is, his character, in this excerpt from his conversation with Macduff in Act IV, Scene iii:
[I] scarcely have coveted what was mine own,
At no time broke my faith, would not betray
The devil to his fellow, and delight
No less in truth than life.
Malcolm defines manhood in terms of personal integrity.
For Macduff, however, manhood is a matter of strength and responsibility. When he learns his family has been slaughtered, he is overcome by grief. Malcolm tells him to "[d]ispute it like a man." Macduff says he will be strong, but he cannot ignore his feelings: "I must also feel it as a man." Macduff's family was murdered as a result of his opposition to Macbeth, and he takes responsibility for their deaths:
They were all struck for thee! Naught that I am,
Not for their own demerits but for mine
Fell slaughter on their souls. Heaven rest them now!
Unlike Macduff and Malcolm, Lady Macbeth does not define manhood in terms of very positive traits of character. For her, being a man means being cruel, this idea made plain in her desire to gain masculine traits to replace her own feminine nature:
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up th' access and passage to remorse
That no compunctious visitings of nature [pity]
Shake my fell [savage] purpose . . . .
Later, she fears that Macbeth will not be cruel enough to follow through with Duncan's murder, and she shows contempt for Macbeth, "so green and pale," when he expresses fear for what they are planning to do.