Macbeth, after talking with his wife, Lady Macbeth, seemed to be 100% in on her plan to kill Duncan and take his position in Scotland as King and Queen. However, while the others were eating, Macbeth is in his chambers thinking. He has many doubts and talks himself out of the plan stating that Duncan was there "in double trust." Duncan is Macbeth's relative (blood is thicker than water, and a relative just does not kill a member of his own family...it trickles down from the Anglo Saxon idea that murdering a brother or member of your blood line is the most horrible of sins), and Duncan is a guest in his own home. Macbeth is a warrior and subject of Duncan, to whom Macbeth has sworn his loyalty. He should be protecting Duncan for all three of these reasons, not planning on killing him. In addition, Duncan has given Macbeth lots of attention, praise, and honors for Macbeth's performance in the war against the Norwegians who were led by MacDonwald and the previous Thane of Cawdor.
So, Macbeth knows that he should be loyal, loving, and willing to risk his own life to protect this man. He decides not to go through with the murder, and even tells his wife that there will be no more talk of it. However, Lady Macbeth is an ambitious one who works magic with words on her husband. She changes his mind in Act I, and the act is committed in early Act II.