I know what the witches told you. You know, that you would become king? Well, don't worry about it, okay? Don't do anything drastic, like, say, kill Duncan. That would be one of the stupidest things you can do. Trust me, I know. I'm from the future, parallel universe, whatever, but do trust me. If you kill him, you'll trigger all sorts of events that will eventually get you beheaded. Do you want to lose your head? I don't think so. And no, I am not a witch.
Would you become king if you kill him? Yeah, but no one would like you. And it wouldn't stop with just him. Guards, comrades, mothers, and children would not be safe from your blood-lust. Can you really put that on your conscience? Please, take my advice, and go about your life as you normally would. I'm sure you'll become king anyway without needing to kill anybody. And be patient. Wait, like, a hundred years if necessary.
I know your wife will pressure you into killing Duncan anyway. Have you ever heard of saying "no" to peer pressure? This is a perfect example of when to break away from it. Would you jump off a cliff just because I kept nagging at you? I hope not. Anyway, she would kill herself from the guilt of the murder she's trying to get you to commit. A good husband doesn't want his wife to kill herself.