In preparation for King Duncan's murder, Macbeth is first pressured by his wife, Lady Macbeth to kill him to become King. Thinking of the royal seat has clouded his mind, making him uneasy and confused about his thoughts. Macbeth feels hesitant and guilty for having these murderous thoughts, because King Duncan had put his trust in him as a kinsman and a subject. Having this trust in him, and him staying at Macbeth's castle made him think that he shouldn't be the murderer, but the one who would fight off the murderers. Also Lady Macbeth is having devilish thoughts and is heavily persuading Macbeth making him change his mind. In the end, he murders the King himself, saying ambition if the only thing leading him. He spends more time and is more careful at deciding and preparing for the King's murder.
In Banquo's case, he was with Macbeth during the encounter with the three witches which makes him skeptical about the sudden murder of the King. Macbeth knows of Banquo's suspicions and he does not want anyone else to know, so he plans to murder him and his son, Fleance. He is afraid of Banquo because of his nobleness and determination to do just about everything. As well as the prophecy that said that Banquo is the father of the next king in line, Macbeth is in fear that the murder was for Fleance's own good instead of his own, and that he killed a good man and tortured his conscience for someone else to become King. So this time, Macbeth feels no remorse, he is more ruthless and thoughtless about murder. All he wants to do is get rid of any suspicions, and be able to be the next one in line for King. Furthurmore instead of murdering Banquo himself, he sends out 2 murderers, and a mysterious other joins to do it for him.