"Thanks for that. There the grown serpent lies. The worm that's fled hath nature that in time will venom breed." What is the context and significance of these lines from Macbeth?
Remember that the weird sisters, in Act I, told Macbeth and Banquo their fates. Macbeth would be thane of Cawdor and king, but would not sire kings. Banquo would never be king but would sire a line of kings. Once he is crowned, Macbeth become increasingly paranoid, and one of the first things he does is hire murderers to kill Banquo and his son Fleance. The leader of the cutthroats has just come to Banquo's castle to report that Banquo is "safe"--meaning "safe in a ditch," dead--but Fleance escaped.
Macbeth responds: "Thanks for that. There the grown serpent lies." That is, Banquo, the "grown serpent," is dead. He goes on to say: "The worm that’s fled / Hath nature that in time will venom breed; No teeth for th' present." He's here referring to Fleance, who is not yet old enough to breed, as a "worm" (a baby snake in this instance). He says he still is a threat because, in time, he'll have venom (when he's older), but he's no threat right now.
Macbeth has failed to change Banquo's fate, and his son will begin a line of kings, the most recent of which--at the time the play was performed--was understood to be King James I, the new king of England, Scotland, and Ireland. This is the king Shakespeare wrote the play for, so Macbeth is the bad guy and Banquo and his progeny are the good guys.