When Macbeth first hears the prophecies, and when the first 2/3 of it comes true, he does think of killing the king, but also, towards the end of Act 1, Scene 3, he thinks that perhaps he doesn't need to do anything to become the king : "If chance will have me king, why, chance may crown me without my stir."
On the other hand, Lady Macbeth, on receiving the letter telling her about the witches' prophecies, she immediately thinks that she and Macbeth will have to kill king Duncan. She also decides that Macbeth is too nice to kill the king, sayin that he "is too ful o' the milk of human kindness" and when she hears the Duncan will visit their castle that night, she immediately appeals to the evil spirits, to (ironically) give her the strength to kill the king.
In Act 1, Scene 7, Macbeth is doubtfull of Lady Macbeth's plot to kill the king. He doesn't think that he will be able to live with the guilt of killing his king while he is staying under his very roof, and then decides that he will not kill the king. This shows that Macbeth is thinking about what he is going to do, and shows that he does feel guilt and is weighing up the situation, unlike Lady Macbeth who never thinks twice about killing the king.
When Lady Macbeth notices that Macbeth has left the room, she goes to speak to him. Macbeth firmly tells her that they will not kill the king : "we will proceed no further in this business". Lady Macbeth, however, tells him that his love is worth nothing if he refuses to go through with the plan, saying that his love is as accountable as his indecisiveness. Macbeth wants his wife to love him and wants her to trust him, so he agrees to go through with their...