Well my point is that pretty much all of our laws are build around ethic values, which are developed within a society. There is no logical or scientific reason that would make killing other people bad, but we still should have strict rules about this.

Laws are always built around soft things like "what is obscene", "at what point is someone naked in public", "How much alcohol can a drink have before it is a alcoholic beverage?", "did the person die of natural causes, or was killed by some event years ago, that wasn't properly treated."

Society decides what is acceptable and what isn't and that changes through time and culture.

Your argument is therefore not a good one, you have to make a case based on ethics.