Macbeth expresses his inner turmoil, both before and after he murders King Duncan, in famous soliloquies. In Act 1, Scene 7, before he has committed the deed, he thinks of the reasons why he shouldn't go through with it and the probable future consequences.
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 the end-all here,
But here, upon this bank and shoal of time,
We'ld jump the life to come. But in these cases
We still have judgment 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. 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. 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;
And pity, like a naked new-born babe,
Striding the blast, or heaven's cherubim, horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind. I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other.
Then in Act 2, Scene 3, after Macduff has discovered Duncan's body and aroused the whole castle, Macbeth says to himself:
Had I but died an hour before this chance,
I had lived a blessed time; for, from this instant,
There 's nothing serious in mortality:
All is but toys: renown and grace is dead;
The wine of life is drawn, and the mere lees
Is left this vault to brag of.
Shakespeare seems to have been anxious to depict Macbeth as a tragic hero rather than as an out-and-out villain. Like Aristotelian tragic heroes, Macbeth is supposed to have only one character flaw, which is ambition. He does not really want to murder Duncan, but his wife is goading him into doing it, knowing that he wants to be king as much as she wants to be queen. The three witches also seem to have been motivating him to do the deed because it is his destiny to be king of Scotland and he will obviously never have a better chance to fulfill that destiny than when King Duncan, for the first and only time, is a guest in his castle. As Lady Macbeth tells him:
Nor time nor place
Did then adhere, and yet you would make both:
They have made themselves, and that their fitness now
Does unmake you.
What she means is that there was no such golden opportunity to murder the King when they first thought of the idea. Because the battle has brought Duncan to this vicinity, he is forced to become Macbeth's overnight guest along with his sons Malcolm and Donalbain. It would seem that Fate has brought Duncan to Macbeth's castle and given him the once-in-a-lifetime opportunity to realize his ambitions and please his even more ambitious wife.
Since Macbeth is not a legitimate king, he does not know how to behave like one. He is naturally suspected of Duncan's murder because of his guilty behavior and the fact that he was the one who benefitted from the King's death. He alienates the other thanes and has to resort to ruling by terror. Macbeth's inner turmoil evidently has driven him completely mad by the time he is killed by Macduff.