The answer is b. When Macbeth kills Banquo, he no longer hesitates to kill anyone who he deems a threat to his rule as king. Before he killed King Duncan, he was still unsure if he will do so or not until his wife shamed him into killing him. When it came to killing Banquo, he did it out of his own will. This was followed by killing Macduff’s wife and son since he could not kill Macduff himself. He no longer cared so long as he stayed in power.