Macbeth should not kill Duncan to try and assume position as the King. For one Duncan is Macbeth's cousin. "O worthiest cousin", states that not only that Duncan considers Macbeth worthy, but it's not an unknown fact that they are cousins. There is a difference between being a blood cousin or being a blood but a recognized cousin. The crown is already spoken for anyways, "Our eldest, Malcolm, whom we name here after the Prince of Cumberland." So even is Macbeth does kill Duncan, Malcolm would get the throne. Macbeth's grand ploy may not be as great as he thought. Then again the original idea started with the witches getting this horrid idea into Macbeths head. The witches are evil. Hence why would the 3 weird sisters try and give good advice or thoughts to Macbeth? Their evilness is shown throughout the play a number of times. Act One, scene three "Where hast thou been sister?" "Killing swine."
Those lines just prove that they are and care about nothing but there selves. There is no nothing good going to come for Macbeth out of this idea. This is why Macbeth should not try and kill Duncan to assume the position of King.