In Act 1 scene vii we see clearly that Macbeth is troubled with how to proceed knowing that the witches’ prophecies seem true. He opens with indicating he would like the deed over quickly and easily-
If it were done when ’tis done, then ’twere well
It were done quickly.
However he realises that the death of a king will be felt deeply throughout the kingdom, and that his role should be to protect his monarch, not to kill him.
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.
Macbeth muses that Duncan has been loved as a king, and will be sorely missed by his loyal subjects:
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off.
Macbeth has been treated well by Duncan, and has no personal cause to wish him dead, other than the evil seeds planted by the witches. He can see that he will be hated for his crime, and that he will suffer deep discontent in himself for the deed. He reveals the one source that will force him to take this terrible action:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other-