Does Banquo kill one of the murderers to save his son in Macbeth?
No, Banquo does not attack the murderers, he just tells Fleance to run.
Macbeth hires three murderers to attack Banquo and Fleance. He is concerned, because Banquo heard the prophecies and Macbeth knows he might be suspicious. He wants Fleance dead also because the prophecy said Baquo’s sons would be king, and Macbeth wants to prevent that.
Banquo and Fleance are taken totally by surprise. One minute they are remarking on the weather, and the next minute they are being attacked.
Let it come down. [They assault Banquo.](25)
O, treachery! Fly, good Fleance, fly, fly, fly!
Thou mayst revenge. O slave! (Act 3, Scene 3)
Banquo does not fight. In fact, he does not even want Fleance to fight. He tells him to run, and worry about getting revenge later.
Banquo does get his revenge. He comes to the banquet as a ghost, and makes a fool out of Macbeth, who is very disturbed by his being there.
The fact that Banquo and his son were ambushed and did not fight back underscores the terrible treachery of Banquo’s murder. Macbeth did not even have the guts to do it himself. He hired strangers. Macbeth has really turned his back on his friends and his morals. All he cares about is continuing to be king.