Why didn't Macbeth...?

(i'm just wondering) I read Macbeth in my english cl since we have to write an essay on it for our final. It's really a good book like The Odyssey. But what I don't get is, why didn't Macbeth sent someone to kill Macduff (like he did for Banquo) instead of killing his family. What I remember from the book is the witches told Macbeth to fear Macduff, not his family.