Banquo doesn't have much time to react to the attack from the murderers sent by Macbeth in Act 3, scene 3. He does realize, though, that the attackers were sent by Macbeth because his words are: "O, treachery!...." and "...Thou mayst revenge..." (III.3)
By saying "treachery" and by telling his son to get "revenge," he demonstrates he knows that Macbeth is behind the killings. His first words are to tell his son to run away, so his first concern is for his son and his son's safety. This shows us that Banquo is a good man and caring father.
The haunting by Banquo's ghost that occurs in scene 4 of Act III indicates that it truly was treachery for Macbeth to have his good friend killed. Whether the ghost is a figment of Macbeth's guilt-stricken conscience or is a real ghost come to torment Macbeth is not clear. Either way though, Macbeth was a traitor to have his best friend killed for the treachery of saving his ill-gotten crown, and Banquo had fully realized that.