Macbeth wanted both Banquo and his son, Fleance, dead due to the prophecies of the witches. They foretold that Macbeth would be king, but not his sons and that Banquo would not be king, but his descendents for many generations will be king.
Macbeth considers them both threats, but Banquo is compared to the "grown serpent" and the "worm" or immature snake, if you will, is Fleance. Macbeth recognizes that even though Fleance has no power right now, he has the capability of power in the near future, and is still a threat. "The worm that's flead hath nature that in time will venom breed, no teeth for the present."
Interestingly enough, young snakes have teeth and their venom is more potent than the mature snakes' venom since it is more concentrated. I don't suppose scientists of the time knew this, but Shakespeare was correct in having Macbeth predict Fleance's future power as he flees to England and allies himself with Malcolm and the English forces.
Macbeth likens the dead Banquo to a deceased serpent and his son Fleance to a young snake. The use of serpents/snakes is common in this play as a symbols for danger. This metaphor is important because it implies that Macbeth still considers Fleance a threat even though Banquo is dead.
It was Macduff who allies with Malcolm and the English army, in order to fight against Macbeth. Fleance just escapes from the murdererswho killed his father, Banquo.