"O, treachery! Fly, good Fleance, fly,fly,fly!" - has evidence now that M is an evil tyrant, rather than a chivalrous friend he once knew. Banquo curses M for his treachery. B's final words protect his son, showing his fatherly love and desire to protect his bloodline, so that his son can become a future king. Even though B is killed by M. he defeats him by helping his son to survive. This plants doubt in M's mind, not unlike the witches plant ambition with their magical spells. Fleance’s survival starts Macbeth’s downfall as king.