I think Macbeth is jealous because Duncan's children and there children are going to be king before Macbeth. Macbeth will be feeling jealous, ruthless, angry and so cross that he will do anything to get the throne because he might not ever get there. So he kills the king when he is sleeping and blames the murder on the king's children. His wife wants him to be ruthless but when he kills the king he is in deep regret and goes into a whole world of depression. I have felt this way before when someone gets something i really want. I feel the same as Macbeth, i feel jealous, angry and ruthless  (i don't feel the need to kill though).SL