The Torah states: "If one strikes a man and he dies, he is to be put to death" (Exodus 21:12). The Mekhilta explains why this verse is necessary when a similar law already appears elsewhere in Scripture. (Leviticus 24:17) states: "And a man, if he strike any soul of a man, he shall be put to death." If the Leviticus verse already establishes the death penalty for killing, what does (Exodus 21:12) add?
The answer lies in the precise wording. The Leviticus verse says "if he strike any soul of a man." Read broadly, "striking" could include any physical blow, even a slap. A person might conclude from Leviticus alone that any act of violence resulting in contact with another person's body is a capital offense, regardless of whether the victim died.
(Exodus 21:12) corrects this potential misreading by adding a critical qualifier: "and he dies." The death penalty applies only when the blow actually kills the victim. A person who slaps someone, punches someone, or even inflicts serious injury is not subject to capital punishment unless the victim dies as a direct result. The perpetrator "is not liable for the death penalty unless he kills him."
This distinction between assault and homicide is fundamental to the entire framework of Jewish criminal law. Without (Exodus 21:12), the legal system would face an impossible situation: treating every act of physical violence as a capital crime, with no distinction between a fistfight and a murder. The Mekhilta shows that the Torah carefully calibrates the severity of punishment to the severity of the outcome. The act of striking alone does not warrant death. Only the result, the death of the victim, elevates the crime to a capital offense.