There are so many scenes that stand out to me in Macbeth, but Macbeth's soliloquy in Act I scene VII always resonates with me. In this scene we see that Macbeth's fate is not sealed, he could still decide not to kill the king, but as he talks to himself, he convinces that it is a deed that must be done. He voices that there is no reason to kill the king, except for his own motivations. This decision will push Macbeth to decide to commit the crime of killing his king and leader.
Macbeth first speaks to the deed itself (murdering King Duncan) and wishes that it were already over and done with. Then he speculates on the morality issues he has with killing the king as he discusses all the reasons why he shouldn't kill him. He realizes that Duncan trusts him, as his kinsman (his loyal subject) and as his host (since Duncan is staying in his house). As his kinsman and host he should be looking out for Duncan's welfare and not planning his demise. While it's never okay to murder someone, these reasons strengthen the severity of the crime.
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, as Macbeth points out, Duncan is weak and frail. There can't be any honor behind killing a weak, old man. Duncan's been a good king to his people. Fair, honest and equitable with his power. If he had been a poor king, then killing him would be good for the country, but this is not the case. He has no valid reasons to kill the king and, if people realize what he's done, people will be angry with him.
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 cherbuin horsed
Upon the sightless couriers of the air,
Shall blow the horrid deed in every eye,
That tears shall drown the wind.
It is here when Lady Macbeth enters and tells her husband to man up, stop wasting time and get to work.