The Teachings of Jesus, the Turning the Other Cheek

Killing an armed combatant in the context of warfare is not sinful in itself, you may say. But what about the teachings of jesus, the turning the other cheek? Matthew, chapter five, verse 39. He was talking about a personal relationship with the person who offends us. The slap on the face in the first century was an insult and not a threat to bodily harm or death. It's two different things. And we can't compare the same thing.