If the witch sisters...

[Macholy]

had not greeted Macbeth as King of Cawdor why would he not have been so ready to believe he was king?

[Redzeppelin]

The structure of your question is a bit confusing - but I like your topic because I've thought long and hard about why Macbeth kills Duncan. Here's my idea:

The key to understanding the witches influence over Macbeth was not in that they suggested that Macbeth would be king. I believe that the witches' statement had tremendous power for Macbeth because they named a deep, unspoken desire of his - one that he probably had never told anybody about, and probably had never even spoken aloud to himself before. Here's why I think this: first, notice his response to the witches' announcement that he shall be "king hereafter": he is startled into silence. Look at Banquo's response to the announcement: "Good sir [Macbeth], why do you start and seem to fear things that do sound so fair?" BQs question indicates that MBs response is unusual. So what would the typical response be? Well, ask yourself this: if someone walked up to you and said "Hail future president of the United States" how would you respond? You'd probably laugh and say "Yeah, right." But - what if being president of the US had been your greatest, secret desire, one you'd never shared with anybody - and suddenly, out of nowhere, some stranger identified your very dream. Wouldn't your response be to be startled into shocked silence?

Hence - I believe that Shakespeare very subtlely hinted that the witches named MBs secret desire. Otherwise, he might have heard them and said "whatever." He had no idea the witches were supernatural beings until they disappeared, so why be shocked about their announcement? There were crazy homeless people in medieval Scotland too, you know. I don't think anybody would undertake to kill the God-ordained king of Scotland just because someone said "Hey, you'll be king someday" unless being king was already something that that individual had already thought deeply about, dreamed about.

Don't know if that answered your question or not.