By what the play shows, Macbeth kills Duncan out of ambition, pushed by his wife: in that first meeting the witches only say that he will be king, and it is he who thinks of murder.
On the heath (Act I, Scene 3), the witches hail Macbeth as a future king, without saying how. When the title of Cawdor confirms the second greeting, he is already speaking of a thought that "embraces a murder no more than imagined," and soon says that if Chance makes him king, it may crown him without his having to act. In Scene 4, Duncan names Malcolm his heir, and Macbeth says in an aside: "This is a step I must climb; otherwise I stumble, for it lies in my path."
In Scene 7, alone, Macbeth lists the reasons against the crime: he is Duncan's kinsman and subject and his host, and the king is honorable in how he governs. He says his only mount is "this vaulting Ambition." When Lady Macbeth arrives, he draws back: "We will proceed no further in this plan." She accuses him of cowardice and lays out the plan to get the chamberlains drunk, and he decides: "I am now resolved."
Later, in Act III, Scene 5, Hecate tells the witches that Macbeth "loves his own ends and not you." The witches never say how or when he will be king, and the play never shows whether the crown would have come without the murder.