What is interesting about the way that Macduff swears vengeance upon Macbeth in my mind is the role of Malcolm in manipulating him towards expressing his vow of revenge. Note the way that, at first, after he has heard the terrible news. Macduff is shocked and repeatedly asks if all of his children were killed. At first, he berates himself for their deaths, because he deserted them. However, note what Malcolm says to him to bring him out of his grief and to move him towards thoughts of murderous vengeance:
Be this the whetstone of your sword: let grief
Convert to anger; blunt not the heart, enrage it.
Note Malcolm's role in encouraging Macduff to use his grief to spur on his desire for bloody vengeance, which Macduff does in lines 232-234:
Bring thou this fiend of Scotland, and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!
Thus it is that Macduff sets himself up against Macbeth as his nemesis, which prepares us for the drama of the final act.