Most fundamental Christians believe that the guilt of original sin bars them from entering Heaven, not that it warrants their death. They believe that it is only through the grace of God that sinners (i.e. everyone) can enter Heaven. This is why you don't see them going around murdering people.
I had the conversation that spawned this post with a devout christian. My response wasn't about turning the other check, but about the adulteress that was about to be stoned. There was a circle of men around her, prepared to stone her for adultery. Jesus stepped in (like the bad ass motherfucker that he was) and said, "let ye without sin cast the first stone." -I said the to the guy I was talking with that if he wanted to kill people as punishment, first he needed to find someone without sin to pull the lever. crickets.....
Not sure, tbh. I was just arguing that fundamental Christians can still see 'innocent life' in the sense that they do not deserve whatever has happened to them, while still acknowledging that none are worthy of entering Heaven by themselves.