What does Macbeth do when he learns of Macduff's mission to England?

He decides to slaughter Macduff's family and everybody who traces in his line. Basically Macbeth goes to Fife and slaughters everybody in the castle including L. Macduff and Macduff's son.

In scene 3, a mysterious third murderer joins Macbeth's two henchmen. Why might Shakespeare have chosen to hide the man's identity?