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...

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.