The Torah requires that for a killing to be classified as murder — and thus subject to the death penalty — the blow must be struck in a place on the body where it could actually cause death. The Mekhilta derives this from (Deuteronomy 19:11): "And he lie in wait for him and he rise up against him and he smite him mortally."
The word "mortally" is the key. It tells us that not every act of violence, even premeditated violence, qualifies as murder. The blow must land in what the rabbis call "a locus which is critical to life" — a vital area of the body where a strike of sufficient force could reasonably be expected to kill.
This principle has two components, and both must be present simultaneously. First, the weapon must have "killing potential." A soft object, a light instrument, something incapable of inflicting a fatal wound — these do not qualify, no matter where the blow lands. Second, the blow must be aimed at a vulnerable part of the body. A strike with a deadly weapon to a non-vital area might cause injury, but it does not meet the Torah's threshold for a capital offense.
The Mekhilta's insistence on this dual requirement — deadly weapon plus vital target — protects against the unjust application of the death penalty. A person who strikes another with a heavy stone to the arm might be guilty of assault, but not murder, even if the victim eventually dies. The Torah demands proof that the attacker targeted a part of the body where death was a foreseeable outcome.
This careful calibration reflects the rabbis' deep reluctance to impose capital punishment. Every possible safeguard is built into the definition of the crime, ensuring that the death penalty is reserved for the clearest and most deliberate acts of lethal violence.