Some people simply do not deserve to live after committing crimes of horrible nature. Murderers (especially serial killers), repeat or child rapists, tortures, the people who commit these sort of crimes simply do not deserve to live anymore. When considering taking the life of another person, one should understand that doing so risks getting their own life taken, hopefully a deterrent. They say death row doesn't deter people, but almost every inmate you hear about on death row is begging like a salesman to be spared. Murderers who killed when they were over 18 or 21 should automatically be given the death penalty, murders would probably decline if people knew their own life would end early if they murdered others.
Although it is wrong for the person to commit a crime, it is even more wrong to kill him/her. If the crime committed is murder, then it will be even more ironic to kill him/her-it is like saying the police commit murder on the murderer of an innocent citizen. So the police are murderers themselves, because like the person who committed the crime, they have a reason for doing it.