Macbeth realizes that killing the king will have several consequences. He realizes that his soul will be damned for the murder and he will pay a very harsh price in the afterlife. In addition, the murder of the king will only cause more violence, some of which will be directed at Macbeth because one, he is the murderer, and two, he would now be the king.
In addition to these consequences, Macbeth recognizes that Duncan has been a good king and has shown Macbeth nothing but respect. In fact, Duncan has just rewarded Macbeth with the title of Thane of Cawdor. Macbeth realizes that the only reason he has to kill the king is his own ambition. So, as a subject of the king and as his host while Duncan visits, Macbeth should protect the king from harm, not do harm to him.
Macbeth says that he should not kill Duncan because they are related, because Duncan is his king, and because he (Duncan) is Macbeth's guest, and as host Macbeth should protect him. He also says that there will be justice against such an action, as well as simple revenge. (Someone might kill him the same way.) Since Duncan is in this situation good and innocent, even angels might cry out against the crime. Everyone will find out, Macbeth says, and what makes it worse is that he doesn't really have a valid reason for killing Duncan.
Macbeth realizes that the only reason me is murdering Duncan is because of his vaulting ambition. He is greedy for power although the soliloquy of Macbeths concious is telling him otherwise. In Act 1 scence 7 it shows Macbeth doubts towards the murder. Macbeth knows that the king has always been nothing but loyal towards him, and having rewarding him with Thane of Cawdor, Macbeth calls the whole thing off. Lady Macbeth persuades Macbeth saying he is a coward, no man. His wifes feine strenght is what drives Macbeth even more. So when Duncan stayed the night Macbeth had already placed a dagger a number of times through his chest.