The true reason is unknown and only inferences can be made. It is possible that Macbeth feared that the other two murderers would not do as he told them and MacBeth sent a third murderer to keep and eye on them. He may have also thought that two murderers alone would be unable to carry out the killing of Banquo and after realizing the fact he decided to send a third. If this was the case then MacBeth was partially right because although the murderers were able to kill Banquo his son, Fleance, escaped.
Also, there's a theory that Macbeth himself was the third murderer
In Shakespeare's play "Macbeth," the identity of the third murderer is never explicitly revealed. Some interpretations suggest that the third murderer could have been hired by Macbeth, while others speculate that it could be a random character added to create suspense and mystery.
No one really knows who the third murderer is exactly.
Some say it is Macbeth in disguise because he probably wouldn't leave two unknown people to handle a murder that can possibly damage his future. Also he can make sure nothing goes wrong or " no rubs nor botches in the work". The reason why he is in a disguise is so Banquo won't reveal his knowledge and ruin Macbeth.
On the other hand the other two murderers ask who sent the third, the third replies with "Macbeth." (unless he is lying or equivocating. If it was Macbeth, he might mean that he sent himself.) In their interview, the two murderers specifically are told to keep Macbeth blameless [Act 3 Scene 2 Line 132-133]. Also, Macbeth seems upset by the First Murderer's news of the escape of Banquo's son, Fleance [Act 3 Scene 4 Lines 21-25]. Equally does he seem unknowing [Line 25], and then relieved [Line 28], of the throat-slitting end to Banquo. It appears that Macbeth moves from his interview of the two murderers, to his conversation with his wife, and then to their banquet. In fact, it's to the banquet that the First Murderer brings news of the murder. Do the events allow Macbeth time to dress in a disguise, exit and re-enter the palace, get dressed, and greet his guests?
One idea is that the third murderer is Donalbain (King Duncan son). That's because after Donalbain left they never mentioned anything about him again. Macduff runs off to England to find Malcolm ( the King's other son) but Macduff never goes to find Donalbain. However, the 'son' was never specified, hence the son could have been Malcolm or Donalbain.
Some people say it is Lady Macbeth trying to figure out what Macbeth is intending to do. Some sources say it is one of the three witches, Hecate, or perhaps Fate itself. It is more probable that the third murderer is Seyton (Macbeth's servant) or the Porter or someone else we haven't heard of before.
The third murderer really emphasizes the idea of '3'. Act 3, Scene 3. 3 witches. The Holy Trinity. The three fates. 3 of royal blood, Duncan, Malcolm, and Donalbain. The three levels, heaven, hell, and earth. 3 prophecies. 3 apparitions. Perhaps Shakespeare just felt the need to add a third character.
He is trying to hire them. He wants them to bump off Banquo. But his conscience has trouble with the idea that these are just hired goons, so he dwells on the idea that they have a personal grudge against Banquo. Maybe they do, and maybe they are playing along because the customer is always right, especially when he's a king, but the fact is that these guys are prepared to do anything and that is exactly why Macbeth is hiring them.
It is possible that Macbeth feared that the other two murderers would not do as he told them and Macbeth sent a third murderer to keep an eye on them. He may have also thought that two murderers alone would be unable to carry out the killing of Banquo and after realizing the fact he decided to send a third. If this was the case then Macbeth was partially right because although the murderers were able to kill Banquo his son, Fleance, escaped.
TO make sure that it was carried through. Some even think that the third murderer was Macbeth
It means that Macbeth is so paranoid he doesn't even trust the murderers.
There is no concrete evidence in the play to suggest that the third murderer is Macbeth. The presence of the third murderer serves to show the extent of Macbeth's paranoia and how easily he manipulates others to do his bidding. It adds to the atmosphere of deceit and betrayal in the play.
The murderers hired to kill Banquo are not named. When the time comes to do the deed, a third murderer joins them. Some speculation has taken place as to who this third murderer might be. Some even suggest that his name might be Macbeth, but this would not make sense since Macbeth seems surprised at the feast when the murderer reports that "Fleance is scaped"
Lennox Suggest that Macbeth is both a Murderer and a Tryant
If by Macbeth's murderer you mean Macduff (although it wasn't murder because he killed him in a fair fight) he aligned with Malcolm the elder sun of King Duncan.
Macbeth changes from a war hero in to a evil murderer because all he wants to do is claim power of the throne Macbeth changes from a war hero in to a evil murderer because all he wants to do is claim power of the throne
Blood is used as a symbol representing guilt. It is seen on the hands of Lady Macbeth in 5.1, on Banquo's ghost and the face of his murderer (Macbeth sees it, though the murderer does not notice - Macbeth feels guilt, he does not) in 3.4, and the dagger that Macbeth sees in 2.1, when Macbeth wrestles with the guilt for the deed he is about to do
Both Macbeth and his wife plotted this murder. The target was to be one of Macbeth's closest friends..... he had it setup so that while his friend and his friend's son were traveling to meet him they would be attacked and killed by robbers so that no blame could pass upon the now King Macbeth. The unfortunate son escaped with his life.... the father was not so lucky
The murderer calls Macduff a traitor because Macduff has fled to join Malcolm, the rightful heir to the throne, in trying to overthrow Macbeth. Macbeth sees Macduff's actions as a betrayal of their previous loyalty to him as king.
The play doesn't say so, but it is a plausible interpretation. Macbeth wants to be safe; it wouldn't be safe having these cutthroats who know he was involved in Banquo's death hanging around.
The cast of Macbeth - 1946 includes: William Bartholmay as Macduff David Bradley as Macbeth Grosvenor Glenn as Second Murderer Louis Northop as Duncan Jain Wilimorsky as Lady Macbeth