I think that having Macbeth kill king duncan was a way of contolling Macbeth. Ladt Macbeth doesnt kill DUncun herself because she knows that after Macbeth had killed the king. He would fill the overwelming power of guilt. And she knows she can controll him, beaucse of this overpowering guilt he would feel. This Infintly shows that she is very controlling. - I hope this helps!
