In Act I, what are the things that tempt Macbeth and what are the things that are holding him back when it comes to killing the king?
The first time that Macbeth considers killing Duncan in order to become king occurs right after Angus delivers the news to Macbeth and Banquo that Macbeth has been named the new Thane of Cawdor, making true that prediction from the witches. Macbeth then wonders if the other prediction - that he will be king - will come true. He knows that in order to make that prediction occur, the current king would have to die. Macbeth says the thought of Duncan being killed makes his heart beat loudly and the fears of what would happen if he killed a king make him shake. He again thinks of killing Duncan in Act 1, sc. 4 when Duncan announces that Malcolm has been named the new Prince of Cumberland. Macbeth says that this is a step "on which I must fall down, or else o'erleap,". Then, after Lady Macbeth tells Macbeth that they should kill Duncan that very night since he is visiting in their castle, Macbeth wavers. In Act 1, sc. 7, Macbeth speaks to us in a soliloquy about his thoughts. He says that if he could kill Duncan without fear of reprisal, or without causing problems, or even without facing sure and eternal damnation, then he'd do it without a pause. He goes on to say that he has no real cause to murder Duncan, besides his own ambition, because Duncan is a good king, he's been good to Macbeth, Duncan is a guest in Macbeth's home, and Duncan is a relative. All good reasons NOT to kill Duncan. The only reason to kill him is so that Macbeth can become king which Macbeth realizes could be the source of his undoing.