I assume you are referring to Macbeth's two soliloquys in Act I scene 3 and then Act I scene 7. Both of these soliloquys are very important because they reveal how Macbeth feels about the potential of coming king, but also his feelings about committing murder to be able to achieve that position.
The first soliloquy comes in Act I scene iii, after Macbeth and Banquo have received their prophecies from the witches, and after Macbeth has just found out that the first prophecy is correct - he has been made Thane of Cawdor. Macbeth debates the prophecies of the witches, saying:
This supernatural soliciting
Cannot be ill; cannot be good:
Macbeth goes on to discuss his reactions to the prophecies and the withes. Yes, they were correct in their first prophecy, but does that mean they will be correct with the rest? Also, what is highly interesting in this soliloquy is that something of his inner-mind seems to be reflected in the witches' words, and this exposure of his inner desires really pains him - perhaps he is confronting his own evil ambition that has made him consider foul crimes in order to achieve his goal before:
If good, why do I yield to that suggestion
Whose horrid image doth unfix my hair,
And make my seated heart knock at my ribs,
Against the use of nature? Present fears
Are less than horrible imaginings.
This first soliloquy then reveals a deeply disturbed Macbeth, and hints that becoming King is something he has already thought about.
The second soliloquy in Act I scene 7 presents us with a Macbeth who is thinking through the consequences of his actions. Is it worth killing Duncan? What could happen to him? How will heaven judge his actions? Macbeth says that under the circumstances he would risk eternal damnation to become king, but he might face judgement in this world. He also mentions that Duncan is his guest and leige, and so he is in Macbeth's castle on "double trust". Lastly he says that Duncan is so good a King that the crime of killing him would be much worse for the murderer. He ends his soliloquy before he is interupted by Lady Macbeth with these words:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on th'other -
Here Macbeth concludes that the only motive he has to commit the crime is ambition. But ambition can be compared to a rider who jumps to hard to mount his horse that he falls on the other side. Macbeth provides us with a real insight to his doubts and concerns here - is ambition by itself enough to encourage him to commit regicide and face the consequences, or will it carry him into disaster?
Both of these soliloquys then show us a Macbeth who is disturbed inwardly by the thought of killing Duncan. Act I scene 3 shows us a more ambitious Macbeth, whereas Act I scene 7 presents us with a Macbeth who is fraught with doubts and concerns.