What is the significance of the old man in Act 2 Scene 4 of Macbeth?

In Act 2 Scene 4 of Macbeth, an old
man enters with Rosse after the news of King Duncan's death has spread and Malcolm and
Donalbain have fled from Scotland.  The old man is significant because he is
representative of times past.  He says that over the course of time, he has seen bad
things happen in Scotland, but the murder of the king and the situation surrounding it
is much worse than anything he has ever known:  ". . .but this sore night hath trifled
former knowings" (II.iv.3-4).  At the end of the scene, the old man leaves Rosse and
Macduff with a bit of foreshadowing:  "God's benison go with you; and with those that
would make good of bad, and friends of foes!" (II.iv.40-41).  This paradox is a warning
that all is not what it seems, and the men need to be careful whom they
trust.