The Torah declares that a person who strikes and kills another "shall be put to death" (Exodus 21:12). The Mekhilta immediately qualifies this statement with a critical procedural requirement. The death penalty can only be carried out after the accused has been properly forewarned by witnesses. No one can be executed for murder without prior warning.
This rule might seem counterintuitive. A person commits murder. The act is witnessed. Why would the killer need to have been warned beforehand? The Mekhilta addresses this directly. You might read "he shall be put to death" as an automatic consequence, meaning that anyone who kills, regardless of circumstances, faces execution. Perhaps even a killer who had no idea his actions were capital offenses should be put to death.
The Torah answers with (Deuteronomy 17:6): "By the mouth of two witnesses shall one be put to death." The phrase "by the mouth" is understood by the Sages not merely as testimony after the fact, but as forewarning before the act. The witnesses must have warned the perpetrator that his intended action was a capital crime, and the perpetrator must have acknowledged the warning and proceeded anyway.
Without this forewarning, there is no capital liability. A person who kills without having been warned by witnesses that the act carries the death penalty cannot be executed by the court. The killing may be a terrible act, but the court's power to impose the ultimate punishment requires the procedural safeguard of prior warning. This requirement made capital punishment in Jewish law extraordinarily difficult to carry out in practice, ensuring that the death penalty remained a theoretical deterrent rather than a routine punishment.