It is difficult to say what Shakespeare meant to convey by having Macbeth employ assassins to kill Banquo. Some possibilities might include:
- Banquo is Macbeth's friend, and he is still at a point in the play where he feels remorse for his actions, though he shows none at the time. This is why, of course, he sees Banquo's ghost at the banquet table after the murder.
- Macbeth is king at that point in the play, and does not want to risk his life to have Banquo murdered. As king, he would also be able to employ assassins to do his "dirty work" for him.
- Macbeth's intent was to have both Banquo and his son Fleance killed, to eliminate the possibility that he could fulfill the prophecy by having a line of Scottish kings among his progeny.
- Shakespeare may have simply wanted to show that Macbeth was still a little weak in courage.
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..