There is quite a bit of literary debate about the identity of the Third Murderer in Act 3 Scene 3 of Macbeth.
While the question is not clearly answered in the play, who do you think the Third Murderer might be? Explain. (Act 3)
In Shakespeare's Macbeth, the character who is the best candidate for being the unidentified murderer who joins the others to kill Banquo and attempt to kill Fleance is Seyton, Macbeth's only loyal confidante by the close of the play.
The Third Murderer cannot be Macbeth, because, later, Macbeth has to ask the murderer who reports on their mission whether or not Fleance is dead. Macbeth then reacts negatively and with disappointment when he is told that Fleance escaped. I'm not sure how anyone argues that the Third Murderer is Macbeth. It just doesn't make any sense. Surely the Third Murderer is one of Macbeth's regular henchmen, like the ones he sends to kill Macduff's family.
Seyton seems a good candidate, although naming him is purely speculation. He brings news of the battle to Macbeth, tells Macbeth about his wife's death, and brings Macbeth's armor to him. He is the only person the audience sees who is still loyal to Macbeth in Act 5. Some productions portray him as almost a body guard of Macbeth's, as well as what we today might call an executive secretary who protects Macbeth's privacy.
Of course, someone whose name sounds like "Satan," would certainly make a good murderer as well. The Third Murderer is there to check up on the murderers, not to kill Banquo and Fleance himself. A trusted follower like Seyton seems a good candidate.
The thing is, though, that the identity of the Third Murderer doesn't really seem worth debating. Shakespeare doesn't identify him, and doesn't attempt to create a mystery about it, either. There's no suggestion that knowing the murderer's identity is essential, or that it is a mystery the audience should attempt to figure out.
I have read several "theories" on who the third murderer might be. Some argue that the third murderer is Macbeth himself because he is so worried that something will go wrong that he steps in make sure that all goes as planned. I do not see how this could be so because the murderers both met with Macbeth previously so they obviously know what he looks like, plus when Fleance gets away, the third murderer does nothing to stop him (I think that Macbeth would have tried to stop him). Plus, on the evening of the murder, Macbeth is at the dinner party with many guests.
Another theory is that Macduff is the third murderer. Some claim that Macbeth must have sent Macduff to make sure that all goes as planned because Macbeth trusts Macduff. Plus, Macduff is absent from the dinner party. I do not think this is the case either because Macduff seems to suspect that Macbeth is up to no good after King Duncan is murdered. Macduff tells the old man that he is not even going to the coronation ceremony, and instead Macduff heads home to Fife; this would account for his not going to the dinner party either. Plus, Macbeth does not even tell Lady Macbeth about his plans to kill Banquo--why would he tell Macduff?
It is actually my thought that the third murderer is a supernatural element (maybe Hecate?) that is present during the murder. Neither of the two murderers knows who this person is and when the action begins, the third murderer does not actually take part in the crime. He/she is like a bystander in the scene, only commenting on things as they happen.