Macbeth wanted to dispose of Banquo and Fleance for more than one reason, but he didn't want people to think he was responsible for their deaths. That is why when he sees Banquo's ghost occupying his own place at the banquet table, he says
Thou canst not say I did it. Never shake
Thy gory locks at me. (3.4)
and he proceeds to reveal his guilt to the entire assemblage. Once it becomes apparent that he was responsible for Banquo's murder, everyone will naturally suspect that he was also responsible for the murder of King Duncan. There must be many who already suspect that because of the principle of qui bono? (Who benefits?). Macbeth is the one who benefited from Duncan's death; therefore it is probable that he had something to do with it. From this point on he is losing the affections and support of all the important men in the land and gaining the reputation of a tyrant, and his ultimate downfall is inevitable..