Macbeth, not Duncan, is the most respected man in Scotland. The wounded captain calls him:
‘…brave Macbeth, well he deserves that name…’
Macbeth has defeated two threats to Scottish security in a brief space of time: the rebels from the West under the control ‘the merciless Macdonald’ and the king of Norway who was attempting to invade in the East. Macbeth’s mental and physical strength is such that he defeats both in a short space of time.
According to Macbeth Scotland needs a strong king and Duncan’s sons are untested and inexperienced (‘What’s the boy, Malcom?’). If it were to come to an election in the event of Duncan dying suddenly Macbeth would be a strong candidate to take over the kingship by election, as in fact happens when Malcolm and his brother disappear after the murder of Duncan. He has already thought about becoming the next king, as his immediate reaction to the witches’ prophecies indicates:
‘Good sir, why do you start and seem to fear/ Things that do sound so fair?’ (Banquo)
The witches seem to have a telepathic knowledge of his desire to be king and their first prophecy, that he will be Thane of Cawdor, is immediately confirmed by Ross:
‘He (Duncan) bade me from him call thee Thane of Cawdor’
At this point Macbeth decides to wait and see what happens:
‘If chance will have me king, why chance may crown me without my stir’ He sends a letter to his wife, only hours before his arrival, about what the witches had told him. Macbeth and Lady Macbeth seem to have discussed the possibility of Macbeth becoming king:
‘Thou wouldst be great…’
Lady Macbeth thinks Macbeth wouldn’t kill Duncan because he would prove too honorable to do such a thing even if it meant becoming king soon after:
‘…Yet do I fear thy nature/ It is too full o’th’milk of human kindness to catch the nearest way’
She is more ruthless than her husband and takes the initiative in scene 5, when he returns to their castle. Her advice to him is to be hypocritical:
‘Look like the innocent flower but be the serpent under’t’
When Macbeth starts to waver in Scene 7 she starts by calling him a coward and then overcomes his reservations and fears by telling him how straightforward the crime will be and how easy to arrange.
I think the greatest influence on Macbeth is his wife. Macbeth wants to be king but wouldn’t be able to do it on his own. The witches’ prophecies and the way one of them comes true immediately is convincing for Macbeth but ultimately it is Lady Macbeth who sways Macbeth to carrying out the brutal murder.