Shakespeare reveals Macbeth's complex feelings about killing Duncan through a series of asides in Act 1, Scenes 3-4. An aside is a dramatic convention where a character speaks either to himself, another character, or even the audience, and no other character hears what is said even though they may also be on stage.
The following aside allows the audience to see Macbeth's initial unwillingness to kill Duncan in order to take his place as king. Just after he is named Thane of Cawdor, Macbeth speaks to himself, saying,
If chance will have me king, why, chance may crown meWithout my stir.
[....] Come what come may,Time and the hour runs through the roughest day. (1.3.157-164)
Since he became Cawdor without having to do anything, he thinks perhaps he can become king in the same way. He then says that even the longest day comes to an end; so, even Duncan's reign will end at some point, and then, perhaps, Macbeth can step in without having to do anything violent. Because an aside, spoken to himself, would reveal Macbeth's innermost thoughts and feelings, we can assume that Macbeth really does not want to kill Duncan.
However, once Duncan names his son, Malcolm, the Prince of Cumberland (heir to his throne), Macbeth knows that he will not be next in line. Again, in an aside to himself, he says,
The Prince of Cumberland! That is a step On which I must fall down or else o'erleap, For in my way in lies. Stars, hide your fires;Let not light see my black and deep desires. (1.4.55-58)
In other words, he will either have to stop his progress to the throne here because Malcolm has been named the heir, or he will have to find another way to become king besides being named the heir. He then speaks to the stars, asking them to hide their light so that no one will be able to see the dark thoughts he's now having about what might be required to become king.
He finishes the aside by saying, "The eye wink at the hand, yet let that be / Which the eye fears, when it is done, to see" (1.5.59-60). He won't let himself look at what his hand is doing, and yet he's still going to do whatever it is that will make his eye afraid to look. He's going to murder Duncan. He might feel a little horrified at himself for doing it, but he's going to do it anyway.
Asides, like soliloquys, give the audience an opportunity to hear the character thinking aloud, especially when the character is speaking to himself. We see Macbeth's quick transition from willing to wait and see, to allowing his ambition to guide his actions, however wrong he knows those actions to be.