Soldiers execute evil killers every day. It is the same thing. If someone is a vile, evil killer, serial killer, rapist, torturer, he deserves the same sentence he gave his victims.
Indeed. The world is not as we would wish, and in part it is because injustice is not punished. Many criminals avoid a punishment consistent with their crime. Sometime, the innocent perish, but for the criminal not to get his due is something much more common.