The ten commandments say thou shall not kill, so, for Christians, where is the ammendment that says it's OK to kill killers. I'm referring mostly to the death penalty, but it could apply to warfare as well.
Those Christian writers who discussed capital punishment during the first three centuries after Jesus' execution were absolutely opposed to it. Christians were instructed to not execute a criminal, to not attend public executions and even to not lay a charge against a person if it might possibly eventually result in their execution. 1
One example is Lactantius (260 to 330 CE) who is primarily known for his books "Introduction to True Religion" and "The Divine Institutes." He wrote in The Divine Institutes, Book 6, Chapter 20:
"When God forbids us to kill, he not only prohibits the violence that is condemned by public laws, but he also forbids the violence that is deemed lawful by men. Thus it is not lawful for a just man to engage in warfare, since his warfare is justice itself. Nor is it [lawful] to accuse anyone of a capital offense. It makes no difference whether you put a man to death by word, or by the sword. It is the act of putting to death itself which is prohibited. Therefore, regarding this precept of God there should be no exception at all. Rather it is always unlawful to put to death a man, whom God willed to be a sacred creature."
Those Christian writers who discussed capital punishment during the first three centuries after Jesus' execution were absolutely opposed to it. Christians were instructed to not execute a criminal, to not attend public executions and even to not lay a charge against a person if it might possibly eventually result in their execution. 1
One example is Lactantius (260 to 330 CE) who is primarily known for his books "Introduction to True Religion" and "The Divine Institutes." He wrote in The Divine Institutes, Book 6, Chapter 20:
"When God forbids us to kill, he not only prohibits the violence that is condemned by public laws, but he also forbids the violence that is deemed lawful by men. Thus it is not lawful for a just man to engage in warfare, since his warfare is justice itself. Nor is it [lawful] to accuse anyone of a capital offense. It makes no difference whether you put a man to death by word, or by the sword. It is the act of putting to death itself which is prohibited. Therefore, regarding this precept of God there should be no exception at all. Rather it is always unlawful to put to death a man, whom God willed to be a sacred creature."