Also, you've hit upon the main reasons for Macbeth's downfall pretty well already there. He is already a pretty adept and cold blooded killer at the start of the play, and although some suggest that the witches are the main reason for Duncan's murder you don't see Banquo getting involved (although in historical fact he did). Macbeth regards masculinity as being cold and clinical, hence lady Macbeth's taunts and e way in which he persuades the two murderers to kill Banquo later on.

Look to Macduff as a contrast in terms of masculinity. He deals with the death of his wife and children in a much more balanced and emotional way than Macbeth does, "and I must feel it like a man".