Sparklekid has the most important quote in the answer given previous. In Act I, scene vii comes a very famous soliloquy in the play. I would say that there are random quotes peppered throughout the scenes that lead up to this soliloquy, but this is the first moment that Macbeth is onstage alone to confide his inmost thoughts to the audience. The whole of the speech is the "unfolding" of "his mind" "into murderous intentions towards King Duncan." He says:
If it were done when 'tis done, then 'twere well
It were done quickly; if the assassination
Could trammel up the consequence, and catch
With his surcease success; that, but this blow
Might be the be-all and end-all here,
But here, upon this bank and shoal of time,
We'ld jump the life to come.
In these opening lines of the speech, Macbeth admits that, if he could murder Duncan and be assured of no consequences for his action, he would do it without pause. But, he knows that the world does not work this way. He continues:
. . .But in these cases
We still have judgement here: that we but teach
Bloody instructions, which, being taught, return
To plague the inventor. This even-handed justice
Commends the ingredients of our poison'd chalice
To our own lips.
So, for Macbeth, the idea of karmic retribution, or the idea of his evil deeds coming home to roost, is a potential reason not to go forward with the murder. He goes on in the speech to list other reasons, but, as good and sound and true as his reasoning is, he ignores it all in favor of his ambition:
. . .I have no spur
To prick the sides of my intent,b ut only
Vaulting ambition, which o'erleaps itself
And falls on the other.
This speech is the best single bit of text from the play that reveals the inner workings of Macbeth's mind, rather than a simple reaction to outside circumstance or another character's words or actions. He speaks this soliloquy alone onstage to only the audience, and it is in this speech that the audience sees the reasoning Macbeth undergoes in deciding to kill Duncan.
Please follow the links below for further analysis of this speech.
"I have no spur to prick the sides of my intent, but only vaulting ambition which o'erleaps itself and falls on th' other"
"If chance will have me king, why chance may crown me without my stir"