According to some death penalty supporters, killing prisoners is the only way to ensure that they will never again commit a crime.
Execution certainly incapacitates a person forever.
A state policy of executing prisoners, however, reflects several false assumptions. First, a state must either execute every prisoner who might later commit a killing or claim the ability to determine accurately which prisoners will and which prisoners will not commit crimes in the future. The former option would result in killings too numerous for civilized society to tolerate. The latter option indicates judicial omniscience which defies both reason and experience. It assumes that law enforcement, judicial, and penal systems are incapable of protecting society without recourse to killing.