The story
After Shakespeare, in Macbeth, act iv, scene 3, where Macduff learns his wife and entire family are murdered: : Ro. Wife, Children, Servants, all that could be found. […] : Macd. […] All my pretty ones? Did you say All? Oh Hell-Kite! All? What, All my pretty Chickens, and their Damme At one fell swoope? The imagery is of a bird of prey ("hell-kite") ransacking a whole nest at one blow, fell meaning "terrible, cruel, savage." In later uses of the expression, the force of the metaphor is reduced or lost.
Etymology from Wiktionary, CC BY-SA 4.0.