What could you say to convince Macbeth not to kill Duncan?
With this question, you know that he is going to kill Duncan. You also must have a few different arguments as to why not. But overall this answer must at least be a persuasion not to kill him by a friend of Macbeth.
Simply that seeing as he has put his faith in the witches, and he has become Thane of Cawdor 'without his stir' as he himself says, there is absolutely no reason to suppose that the kingship will not eventually fall to him. And also, once Malcolm is named Prince of Cumberland, Macbeth has no reason to hope that he himself would be named as king following the death of Duncan. Of course, he manages to convince the other thanes that Malcolm's flight means that he must have arranged his father's death (which also makes no sense).
As both previous posters mentioned, it would be difficult to talk Macbeth about of this. I think the best chance you'd have to convince him to not do it would be to convince his wife. Because she's the one who continually goads MB into this action, she's the one you'd have to try and reason with. Unfortunately, her over-reaching ambition would that difficult, too, but perhaps looking into the future and giving her examples of what bad things may happen as a result of this murder would have been enough for her to put on the brakes.
Perhaps the easiest way to convince Macbeth not to kill King Duncan would be to play on the doubts he already has. In his soliloquy, the speech delivered in Act I, scene vii, Macbeth admits that this assassination will bring with it severe consequences. In addition to damning his soul, Macbeth realizes that this violent act will lead to other violent acts, some in retribution for the king's murder. In addition, Macbeth realizes what a dishonor he would be doing to the man that has just rewarded him with the additional title of Thane of Cawdor. As his king and as a guest in his home, Duncan should be protected by his host, Macbeth.
At one point Macbeth goes so far as to call the whole thing off, but is once again spurred on by his wife. Someone close to Macbeth could probably convince him to abandon his murderous scheme for all of these reasons, plus the fact that the witches never explicitly say that Macbeth must murder Duncan in order to be king when they prophesy in the opening scenes of the play.
First, let me say that I don't think I could. By the time Macbeth kills Duncan, he's already too far gone.
That said, if I had to try, I would try the following.
1) I'd review what Duncan had said about Macbeth (all the positive things).
2) I'd review how Macbeth mistrusted the witches at the start, and what a generally bad idea it is to act on prophecy.
3) I'd review all that Duncan had done for him.
4) I'd review his duties as a soldier, and try to hold him to those.
5) I'd predict the future: that killing a king is a bad precedent, especially if you want to be king.
6) I'd try to get him to wait, hoping for more rationality to return.