Could the third murderer be Macbeth? What are the reasons for "for" and "against" this suggestion?

The third murderer would be hired by Macbeth to make sure the job gets done. By this point in the play Macbeth is pretty paranoid. Macbeth would never do the job himself because he is a coward when it comes to doing dirty work like killing his best friend. Macbeth is too much of a "weasel" at this point in the play to face Banquo himself.