Macbeth: comparing Macbeth to Macduff?

MacDuff is a threat to Macbeth because he was not born to a woman but delivered via Caesarian section. He also is one of the main characters who distrusts Macbeth and suspects that he murdered Ducan. However, I think MacDuff only becomes a real threat to Macbeth after Macbeth kills his family, because it gives him the motive for revenge.