Macbeth justifies killing Banquo and Fleance by claiming that Banquo's descendants are prophesied to become kings, and that Fleance witnessed Macbeth's murder of Duncan. This differs from his premeditation of Duncan's murder, in which he was motivated by ambition and guilt. Macbeth's change in motivation suggests that he has become more ruthless and paranoid.
Justifications for killing Banquo and Fleance:
In Act 3, Scene 1 of Macbeth, Macbeth gives two justifications for killing Banquo and Fleance. First, he claims that Banquo's descendants are prophesied to become kings. This suggests that Macbeth is motivated by fear of losing the throne that he has gained through Duncan's murder.
Second, Macbeth claims that Fleance witnessed his murder of Duncan. This suggests that Macbeth is motivated by a desire to silence any witnesses to his crime.
Differences from premeditation of Duncan's murder:
Macbeth's premeditation of Duncan's murder is motivated by ambition and guilt. Macbeth is ambitious to become king, and he feels guilty about his lack of legitimacy. However, he also has doubts about whether he is capable of committing murder.
In contrast, Macbeth's justification for killing Banquo and Fleance is based on fear and paranoia. Macbeth is afraid of losing the throne, and he is paranoid about the possibility of being exposed as a murderer.
How Macbeth has changed:
Macbeth's change in motivation from ambition and guilt to fear and paranoia suggests that he has become more ruthless and paranoid. He is now willing to commit murder to protect his position, even if it means killing innocent people.
Macbeth's justifications for killing Banquo and Fleance reveal his changing motivations and how he has become more ruthless and paranoid. This change is a direct result of his decision to murder Duncan, and it suggests that Macbeth is on a dark path that will ultimately lead to his downfall.