It was my impression that Macbeth paid the men to commit the murders. We know money speaks to many people regardless of the good or bad involved. When I went back it seems that Macbeth did indeed hire the men but he also made sure they were men that were poor. He convinced them that their lot in life was the fault of Banquo's and that Banquo would do the same to their descendants if he should continue living. This would have hardened their hearts and turned them against Banquo quickly. Of course, Macbeth was pretending to be aghast that they would accuse him being innocent.
