have considered this as condemning himself to eternal damnation. Although at this point in the play, he has begun his downward spiral, it is not until after he kills Duncan that he is fully committed to the paththe witches have foretold for him.

The next reason he is reluctant to kill Duncan is that they are kinsmen, cousins to be precise, and killing a kinsman was considered far worse than killing someone who was not a blood relative in this period. As well as being Macbeth's kinsman, Duncan is his king, and so killing him would be treason. Another reason is that Duncan has been very kind to Macbeth, appointing him Thane of Cawdor, and in fact, Macbeth personally likes Duncan and considers him kind and a good king. 

Finally, Macbeth is a host to Duncan, and in this period hospitality bore with it strong reciprocal debts of gratitude and responsibility. Someone violating these conventions of hospitality would be considered not only unethical but impious. 

Macbeth explains his reasons for his reluctance as follows:

First, as I am his kinsman and his subject,

Strong both against the deed; then as his host,

Who should against his murderer shut the door ...

I take it you are referring to the soliloquy that Macbeth gives in Act I scene 7. This is a very important speech that Macbeth gives because in it he reveals something of his doubt and inner anguish about the act as he begins to have second thoughts. Initially, Macbeth starts off by talking about the best way to do it, saying that if it were done quickly, it might limit the possibilities for reprisals. Macbeth says that, for the glory that he could gain, he would "jump the life to come," as in risk the judgement that he might receive by committing the crime of regicide.

However, Macbeth still fears vengeance. He says that by killing Duncan he would teach "Bloody instructions" that "return / To plage th'inventor." Killing Duncan might find Macbeth himself killed out of revenge, which is of course precisely what happens. Then Macbeth goes on to talk about reasons why he shouldn't kill Duncan. Firstly he is Duncan's subject and secondly his host - Macbeth would break trust in a massive fashion were he to kill Duncan. He also says that Duncan has been such a pious and "meek" king that to kill him would surely bring eternal damnation. Lastly, he uses an unforgettable metaphor to describe his ambition:

I have no spur

To prick the sides of my intent, but only

Vaulting ambition, which o'erleaps itself

And falls on th'other -

He says the only quality he can use to convince himself to kill Duncan is his ambition, which can be likened to someone who is to eager to ride and therefore jumps so hard and so high when mounting the horse they fall completely on the other side.

Thus Macbeth is concerned about a whole series of possible outcomes to committing murder, fearing both judgement in this world and in the world to come. Crucially, however, Lady Macbeth manages to persuade him in the rest of this scene in what must be one of the best examples of manipulation in literature.

At the beginning of act 1, scene 7, Macbeth delivers a soliloquy—a speech given by a character who is alone on stage—revealing his true thoughts and feelings regarding his and his wife's plan to kill the king. In this speech, he outlines all of the reasons he has not to kill the king.

Reason 1: First, he says,

If it were done when 'tis done, then 'twere well
It were done quickly. If th' assassination
Could trammel up the consequence and catch
With his surcease success, that but this blow
Might be the be-all and the end-all here,
But here, upon this bank and shoal of time,
We'd jump the life to come (1.7.1–7).

In other words, Macbeth knows that simply killing Duncan will not make him king. If it would, then he would get it over with quickly. If the murder could be like a net, catching everything (and preventing any unpleasant consequences), then it would be the ultimate action for which Macbeth would willingly risk his soul. He hesitates to kill Duncan because he knows it will only be the first action—and risk—he must take; there will be many more required.

Reason 2: Next, he is concerned that killing the king would be like teaching

Bloody instructions, which, being taught, return
To plague th' inventor (1.7.9–10).

Macbeth worries that, if he kills a king and gets away with it, then he may inadvertently teach others that they, too, can kill a king and get away with it; this would put his own throne and life in danger.

Reasons 3–5: Next, he begins to consider his relationship with Duncan, saying,

He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself (1.7.12–16).

Macbeth is Duncan's relative, his subject, and his host. These are three good reasons, alone, why Macbeth hesitates to murder the king. He ought to protect his cousin, he should be loyal to his king, and he should protect his guest.

Reasons 6–7: Moreover, Macbeth continues,

Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off;
And pity, like a naked newborn babe
Striding the blast, or heaven's cherubin horsed
Upon the sightless couriers of the air
Shall blow the horrid deed in every eye,
That tears shall drown the wind (1.7.16–25).

He recognizes that Duncan has been a strong and virtuous leader, and that, when Duncan dies, his many virtuous qualities and deeds will seem like angels that cry to protest the injustice of his death. Further, everyone will pity Duncan, and all the citizens of Scotland will cry a great flood of tears. In other words, Macbeth hesitates because Duncan is such a good person and leader, and because it will be devastating to the Scottish people if he dies, especially in such a tragic way.

Macbeth hesitates because he has sworn his loyalty as a subject to Duncan, because he is related to Duncan (the bonds of family were strong), because he has been treated well by Duncan and has a good reputation, and because he thinks to himself that if the Fates have said he will be Thane of Cawdor and King (one of them having already come true), that he can wait for it to come to him honorably.

There are several reasons Macbeth hesitates to kill Duncan, and he clarifies these for the audience in a soliloquy in act I, scene vii.

Macbeth begins by saying that he knows there will be consequences for his actions. He suggests that he would not be so anxious about murdering the king if the murder would "be the be-all and the end-all" (line 5). However, even putting aside the inevitable damning of his soul in the afterlife, Macbeth knows that he will deal with repercussions in his mortal life, as well:

But in these cases

We still have judgment here, that we but teach

Bloody instructions, which, being taught, return

To plague th' inventor: this even-handed justice

Commends the ingredients of our poisoned chalice

To our own lips (7–12).

First, as I am his kinsman and his subject, Strong both against the deed; then, as his host, Who should against his murderer shut the door, Not bear the knife myself. Besides, this Duncan Hath borne his faculties so meek, hath been So clear in his great office, that his virtues Will plead like angels, trumpet-tongued, against The deep damnation of his taking-off (13–20) . . .

Macbeth delivers a soliloquy at the beginning of act 1, scene 7, where he outlines a number of his reasons not to go forward with the murder of Duncan. First, he says,

If it were done when 'tis done, then 'twere well
It were done quickly (1.7.1–2).

Here, he refers to the fact that simply killing Duncan won't automatically make Macbeth king. If it were as simple as that, then he'd probably just want to get it over with; but there will be other loose ends that need tied up (for example, the existence of Malcolm and Donalbain, the king's sons).

Next, he references "the life to come," concerned that this action—regicide—will certainly affect his afterlife and what happens to his immortal soul (1.7.7). Macbeth also alludes to his concern that his actions could prompt others to take similar action against him:

But in these cases
We still have judgment here, that we but teach
Bloody instructions, which, being taught, return
To plague th' inventor (1.7.7–10).

He worries that once people see that someone can murder a king and get away with it, they might try to murder him for the same reason he is planning to murder Duncan.

Next, Macbeth considers his various relationships with Duncan, saying,

He's here in double trust:
First, as I am his kinsman and his subject,
Strong both against the deed; then, as his host,
Who should against his murderer shut the door,
Not bear the knife myself (1.7.12–16).

Duncan is Macbeth's relative and his sovereign. Macbeth is also Duncan's host. For all of these reasons, Macbeth should be trying to protect Duncan, not threaten him. On top of these myriad reasons, Duncan is a really worthy king and all-around good person. Macbeth says,

. . . this Duncan
Hath borne his faculties so meek, hath been
So clear in his great office, that his virtues
Will plead like angels, trumpet-tongued, against
The deep damnation of his taking-off . . . (1.7.16–20).

Duncan has been so virtuous and humble that his actions will seem to speak for him after his death, like angels crying out against the horrible nature of his murder. Everyone in the kingdom will pity Duncan so much that his murder will affect "every eye, / That tears shall drown the wind" (1.7.25). The entire kingdom will mourn the loss of this great man.

Finally, Macbeth says that, though he has a great many reasons not to commit the murder, he only has one reason to do it: his "Vaulting ambition, which o'erleaps itself / And falls on th' other—" (1.7.27–28). In other words, only his ambition spurs him on, and he recognizes that it might overshoot the mark and compel him to attempt too much.