This notion of perfection is one that now comes to dominate Macbeth's thoughts. Banquo's death would make Macbeth's "health . . . perfect"; and the crime must be committed at "the perfect'st spy of the time" (the exact hour). Both of these quotations foreshadow Macbeth's line in Act III, Scene 4, when, hearing of the botched attempt to kill Fleance, he remarks "I had else been perfect." The tragic assumption that one can commit a perfect crime and escape the consequences is about to be tested.
As if to impress us with the connection between the killing of the king (the blame for which could, after all, be laid at the door of Fate) and the killing of Banquo (blame for which most definitely cannot), the final couplet ("It is concluded: Banquo, they soul's flight, / If it find heaven, must find it out tonight") ironically recalls the words spoken by Macbeth immediately prior to his killing of King Duncan: "Hear it not Duncan, for it is a bell / That summons thee to Heaven, or to Hell."
