What do you think is the motivation both for Lady Macbeth and Macbeth to commit the murder? I think "greed" is thrown around as the answer, but I wonder if "faithlessness" may also be credited: It is prophesied that Macbeth would be king, what would motivate quickening that prophecy besides impatience, what motivates impatience? We have greed - greed, like faithlessness, indicates a lack or an insecurity in the character. But I just wonder if faithlessness, and the haughty arrogance that we put upon ourselves to avoid that fragility in the face of a known-unknown destiny could just as easily be the answer to why Lady Macbeth got all up in arms about murdering the king.

So, a lot of more modern interpretations focus on the aspect of greed. I think Macbeth is the most (or more) parodied of Shakespeare's plays by modern avant-garde dramatists and playwrights. You could see Eugene Ionesco's Macbett or Alfred Jarry's Ubu Roi, both of which center on ambition, greed, cowardice, both written in context of changing economic and social values post-industrial revolution (Jarry's was pretty good if too politically aggressive, it was just so stupid - I didn't read Ionesco's yet, but I've read some of his work and I think he is just a touch too pompous). I have my own suspicions about how narrowly writers can view their own context - to criticize the change of society and social class without reconciling the ample opportunities presented through technological development, but that's a long tangent that is not fully developed.

As for Macduff, there are a lot of different answers we could consider. The first that comes to my mind is, maybe he considered it was more dangerous for his family to be with him. Most reasonable people would target the man and not the family in the event of these more archaic power struggles.

The fact of the witches is probably why I prefer to think about faith instead of greed when it comes to the baser themes of the play. Shakespeare does deal a good amount with metaphysical and divine themes although considerably less than his medieval predecessors - before Shakespeare's time, what was popular were mystery and miracle plays (some people theorize this is what he watched growing up) which were laden heavily with Christian themes, iconography, and characters. Shakespeare was part of the English Renaissance, so much of his work incorporated classical and "pagan" elements. I think the return to the classics always has something to do with a reaffirmation and realignment to what are the most "high" or "ideal" aspects of humanity, especially following an age of barring access to information, repression, forced conformity. To make the characters witches instead of more regular Christian demons or the devil could say to the audience, yes, we live in a world where this is believed as well, "This happened too." To affirm folk history is to affirm history...anyway...

I also read once that the reference to the witches has to do with the historical aspect of the play, it being commissioned by a king during the time of a witch hunt. But I read this once, somewhere, and I can't find the reference for it, so take it as that.