(Exodus 21:22) "And if men fight, etc." What is the intent of this section? From (Ibid. 14) "And if a man be bent against his neighbor to kill him," we hear only that one who intends to smite his foe and does so is to be put to death; but we do not hear the same for one who intends to smite his foe and smites his friend. It is, therefore, written (to this effect) "And if men fight … and if there be death (in his friend) then you shall give a life for a life."
The Mekhilta Asks Why This Passage Is Necessary
Mekhilta Tractate Nezikin 8:1