What was Banquo's revenge, and how did it affect Macbeth?
It may seem strange to speak of Banquo as taking revenge on Macbeth, as Macbeth kills Banquo and sends his son, Fleance, into hiding. On this account, Macbeth has total victory over his former friend. If, however, we take a broader view of the play's action, we see that it is Banquo, not Macbeth, who is ultimately triumphant.
In Act IV, the witches taunt Macbeth by conjuring a procession of eight kings; the last of these holds a mirror in which the reflection of even more can be seen. Macbeth is led to realize that these are all Banquo's descendants. This is a hard blow to Macbeth, who is childless, and whose reign is quickly unraveling. The message is clear: Macbeth's reign will end miserably, and Banquo, through his descendants, will be victorious. In this way, Banquo will have what Macbeth can never have: children, lineage, and honor.
It could also be argued that, by Act IV (and certainly Act V), Macbeth is increasingly distraught and unable to sleep. He is now responsible not only for the deaths of King Duncan and Banquo, but also Lady Macduff, her young son, and others. Prince Malcolm, Macduff, and an entire army are marching toward him; Lady Macbeth has gone insane. Although Banquo is dead, he is not a murderer, and Macbeth's existence is certainly hellish. Hence, we can argue that Banquo's honorable life (and death) are also a form of revenge against Macbeth, who by the end of the play is utterly defeated, tormented, and alone.