I'm not sure that this play really gives us much insight into the life of a king, although it does give us great insight into the life of a tyrant. Ideally, a king is someone who lives his entire life in order to serve his people and nation. I say "ideally," because obviously this ideal is rarely achieved, although the current Queen of the United Kingdom has not done a bad job at being a conscientious royal. A tyrant, however, someone who is mainly concerning with winning and retaining his own power and whose rule is often destructive and abusive. Macbeth, unfortunately, is far more a typical tyrant than an ideal king.