In this scene, we see that Macbeth has sent another murderer to make sure the killing is done. The murderers do kill Banquo, however, Fleance manages to escape. Banquo asks Fleance to avenge his death.

BANQUO

O treachery! Fly, good Fleance, fly, fly, fly!
Thou may ’st revenge —O slave!
                                   BANQUO dies. Exit FLEANCE

THIRD MURDERER

Who did strike out the light?

FIRSTMURDERER

     Was ’t not the way?

THIRD MURDERER

There’s but one down. The son is fled.

SECOND MURDERER

We have lost best half of our affair.

FIRST MURDERER

Well, let’s away and say how much is done.
                                                                      Exeunt