Macbeth lies to the murderers and tells them that Banquo was responsible for their ruination and downfall. He does this so they have a personal interest in killing Banquo and to try to get them to agree to kill Banquo.
The whole reason Macbeth is going through the trouble of getting Banquo and his family killed is because he is paranoid that Fleance, Banquo's son, will take his throne like the witches foretold in their prophecy.
