Macbeth thinks the murder of Banquo is justified. He does not have any doubts, unlike the murder of Duncan.
Duncan worries about killing Duncan, and has doubts leading up to the murder. He has no such second thoughts concerning the murder of Banquo, even though Banquo is a fellow soldier and noble, and a close friend.
Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be fear'd. (Act 3, Scene 1)
Macbeth feels that his murder of Banquo is justified, because he is king now. He has to protect his crown at all costs. He is angry that the witches prophesized that Banquo’s sons would be king, and is intent to avoid a “fruitless crown.” He is also worried that Banquo knows too much, and might suspect he killed Duncan. So Banquo has to go.
Macbeth expresses his fear and his deep resentment of Banquo as he anticipates murdering him. Before taking off for a pre-dinner ride, Banquo reminds Macbeth, now king, that the witches prophesied that Banquo's sons and grandsons would become kings. He speculates as well that Macbeth did something illicit to gain the crown.
As Macbeth thinks of Banquo's words, he feels affronted and anguished that he has taken such great risks to become king and has Duncan's blood on his hands--only to have Banquo's sons someday take the crown. As Macbeth puts it:
If ’t be so,For Banquo’s issue have I filed my mind;For them the gracious Duncan have I murdered;Put rancors in the vessel of my peaceOnly for them; and mine eternal jewelGiven to the common enemy of man,To make them kings, the seed of Banquo kings!
Fleance, his son, that keeps him company,Whose absence is no less material to meThan is his father’s, must embrace the fateOf that dark hour.