It deters bad people to commit heinous crimes.
One reason supporters of the capital punishment are for death penalty is its effectiveness as a crime deterrent. According to the advocates for death sentencing, potential criminals will be scared to suffer such harsh punishment and as a result, they will be hesitant to commit crimes like rape and murder. In fact, for them, it is the greatest deterrent of criminal acts. If an offender commits a crime that is punishable by death and gets executed, this will be known by the public, including those who are potential criminals. For fear of suffering the same plight, they will be discouraged to commit crimes.