If I were trying to persuade Macbeth not to kill Duncan, I'd try to help him realize the affects the evil act will have on him, and the fact that fate can take care of itself.
Macbeth,
Killing Duncan will give you no satisfaction. It will plague your thoughts and you will feel increasingly guilty until it drives you mad. There are other ways of getting power, just as the witches promised. It's wrong to take your fate into your own hands, and think that you can manipulate your own future. Killing is not befitting of a man of your stature. Duncan has recently rewarded you with another position as thane of Cawdor, and has done absolutely nothing to deserve such a treatment from you. You have gained his trust, and are basically his closest advisor, killing him will only make you regret it for the rest of your life. It also cheapens the act, and makes you think that you can kill people to get what you want. This is not the way to go about your life, thinking you can always get away with this heinous crime. Your wife is twisting your emotions, and you should be able to see that she only agrees with this form of action because she lusts for power. Put her in her place as the subordinate to you, and tell her that it is best to let fate take its own course of action. It's best to just forget what the witches have told you and let your future decide itself.