“FIRST MURTHERER. But who did bid thee join with us?
THIRD MURTHERER. Macbeth.
SECOND MURTHERER. He needs not our mistrust, since he delivers
Our offices and what we have to do”
Macbeth very quickly decides that Banquo must be killed. This seems very strange, because Banquo and him have been good friends, at least for the whole narration of the book. With Duncan and his kids out of the way, what possible reasoning could he have for killing Banquo?
It seems that the paranoia is finally starting to kick in. After killing Duncan, the guilt is starting to kick in. Macbeth’s greatest fear is someone finding out his dark secret. He feels like his deed has trapped him, and until all trace of it is gone, he is in danger.
Because Banquo is very close to him, Macbeth probably sees him as one of his biggest threats. That is why he is suddenly obsessed with plotting the death of his once best friend.
Macbeth could always just tell Banquo the truth, but Banquo would probably expose his secret. So, Macbeth’s nervousness causes him to hire the murderers to kill Banquo. He doesn’t trust Banquo.
That is why he has to hire a third killer last minute. He wanted to be 100% sure that Banquo was dead. That’s why they say “He needs not our mistrust.”
EDIT: This was my 100th post! Yay!