Loss of honor:
Macbeth breaks every code of honor imaginable when he kills Duncan, because Duncan is (a)his king, (b)his kinsman and (c)his guest, and honor demands the protection of all three of those. Macbeth mentions this, his 'thrice damned'-ness, in one of his pre-murder soliloquies.
Then there's everyone else. Young Siward's death and his father's reaction to it in the last act deal with how honor is seen by these macho warrior types. A different perception of honor is given by Lady Macbeth--look at the way she prods Macbeth towards murder. The way Malcom talks about the virtues of a king could be seen as a kind of honor, too.
When Macbeth sends killers to murder Banquo and Fleance Banquo shows a quality that is admirable in any man and needed in any father. As they are attacked Banquo takes it upon himself to fend off the killers while his son escapes. While he is dying he asks his to deliver vengeance upon those who have done this to him "Fly, Good Fleance, fly, fly, fly! Thou mayst revenge." By telling his son to flee rather than stay to help he is ensuring his lineage continues past the ambush, thus doing his last manly deed. However, in telling Fleance to flee he is also telling the boy that when he grows up he must take revenge on the murderers thereby being a man. On the subject of the murderers Macbeth used the same degrading talk to them as Lady Macbeth used on him before he killed Duncan. He convinces them that Banquo is their enemy and that they would be less than men if they did not kill him.
In conclusion, Lady Macbeth corrupted Macbeth into not being a man and then he corrupted other people by the same means that he was corrupted. He accused people of not being man enough to kill people or he tries to corrupt decent men, like Macduff, into doing something that a man would not do. The play, Macbeth, is all about 'being a man' it shows that the phrase means different things to different people. To be a man is so important that...