What's Macbeth's reasoning not to kill Duncan?
FIRST WITCH. All hail, Macbeth! hail to thee, Thane of Glamis!
SECOND WITCH. All hail, Macbeth! hail to thee, Thane of Cawdor!
THIRD WITCH. All hail, Macbeth, that shalt be King hereafter (1.3.50-53)
Macbeth replies that he is already Thane of Glamis. Ross soon arrives from King Duncan to praise Macbeth for his victory over Macdonwald and to tell Macbeth that the King has declared him Thane of Cawdor. The former Thane of Cawdor turned out to be a traitor (coincidentally, so does the new Thane of Cawdor), so the King condemned him to death, and decided to give Cawdor's title and property to Macbeth for his faithful service.
ANGUS. Who was the Thane lives yet,
But under heavy judgement bears that life
Which he deserves to lose. ...
But treasons capital, confess'd and proved,
Have overthrown him. (1.3.116-123)
At first, Macbeth had some doubts about what the Witches told him, but now he's starting to believe them. What the Witches said about being King also stirred up some long-repressed ambitions in Macbeth, as well as some ideas about killing Duncan.
MACBETH. [Aside.] Two truths are told,
As happy prologues to the swelling act
Of the imperial theme!...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... (1.3.138-149)
Macbeth thinks about it a little more, and he has a really sensible idea—his first reason not to kill Duncan.
MACBETH. [Aside.] If chance will have me king, why, chance
may crown me
Without my stir. (1.3.155-157)
After all, Macbeth didn't have to do anything to become Thane of Cawdor. Why does he necessarily have to do anything to become King?
Macbeth might have stopped thinking about what he might do to become King right there, but why didn't he? It's because in the next scene, Duncan seals his own fate.
In act 1, scene 4, Duncan puts an obstacle in Macbeth's way to being King by declaring his son, Malcolm, the Prince of Cumberland, and heir to his throne.
Macbeth decides that the throne isn't going to come to him simply by chance or because of some scraggly old witch's prophecy. If he wants to be King, he's going to have to do something to become King, and that means he's going to have to kill Malcolm, Duncan, or both of them.
MACBETH. [Aside.] The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be
Which the eye fears, when it is done, to see. (1.4.55-60)
Macbeth gets caught up in the prophecy and with the idea of becoming King, and, with some urging from Lady Macbeth, he agrees to kill Duncan.
In act 1, scene 7, with Duncan under his roof, and everybody having a nice dinner, Macbeth steps out of the dining room and takes a moment to think about what he's agreed to do from practical, political, and even moral frames of reference.
Other Educators have discussed the specific reasons that Macbeth raises for not wanting to kill Duncan in his "If it were done when ’tis done, then ’twere well / It were done quickly" soliloquy.
Macbeth's concerns seem to be that he might suffer serious political consequences if he kills Duncan—that what he does will come back to "plague the inventor" (1.7.10)—and that he doesn't want to lose his honors and the good will of the people that he's worked so hard to achieve.
MACBETH. We will proceed no further in this business:
He hath honor'd me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon. (1.7.34-38)
These are secondary concerns, of course. The real reason that Macbeth is reluctant to kill Duncan is what he blurts out to Lady Macbeth when he runs out of excuses about "I dare do all that may become a man..." (1.7.51).
MACBETH. If we should fail? (1.7.66)
Lady Macbeth almost laughs at him.
LADY MACBETH. We fail?
But screw your courage to the sticking-place,
And we'll not fail. (1.7.67-69)
Macbeth decides that if Lady Macbeth is strong enough and determined enough to go through with this, then so is he.
MACBETH. I am settled, and bend up
Each corporal agent to this terrible feat. (1.7.90-91)