In act 3, scene 6, of Macbeth, Lennox comments to another lord on the three sons of the two murder victims. He prefaces his commentary by pointing out that strange occurrences have been happening lately. Lennox brings up "the right-valiant Banquo," who was killed in the street, and his son Fleance, who fled the scene soon after. He says that Banquo was out too late, and "you may say if 't please you, Fleance killed" his father. Lennox is telling us of people's belief that by running away, Fleance has placed suspicion on himself. Lennox concludes by stating that men should not go out walking late at night. He then turns his attention to King Duncan's murder and "how monstrous" Malcolm and Donalbain were to kill their "gracious father." According to Lennox, Macbeth did the right thing by killing Duncan's servants, who, because they had fallen asleep from drinking so much, failed to protect him from his sons. Lennox adds that if Macbeth places the brothers "under his key," meaning in prison, the two murderous siblings will discover the punishment for killing their father, the king. He adds "So should Fleance" as a reminder that Fleance would face the same punishment for patricide.

Lennox lets us know that most people believe Fleance, Malcolm and Donalbain are all responsible for their father's deaths.  However, he is first nobleman to speak up and cast doubt.  He does not believe that these characters are guilty, and instead believes that Macbeth had a hand in it all.