To ascribe abstract laws to Ethics ignores the particulars of any given situation. Take Kieslowski's
Dekalog 5, for example. A young drifter kills a taxi driver as sort of existential experiment. From that "murder is wrong" and that "all are equal before the law" one would conclude that Jacek should be given the death penalty. If we don't begin with an assessment of Ethical truths, then we can take into consideration that he is a misguided youth probably stuggling with Nihilism and that some form of rehabilitation is probably better suited to such a crime. The assumption that there are objective Ethical truths is, rather ironically, resultant in a number of the absurdities of Law.
A methodology that proceeds from that "murder is wrong" could be quite interesting. To what degree is what one is doing like murder? I would, perhaps, deduce that it is wrong to violate what an other existentially attests. I would, then, of course, have to qualify this. You could say that in so far that what an other existentially attests is in good faith and Ethically sound and valid that it is wrong to violate this. I could parcel out an entire ethic from there as you could as well. I don't think that I would start there, though.
As murder is the exception and not the rule, I'm not sure that it would be useful to create an ethic proceeding from that "murder is wrong". We can say that "murder is wrong", but that is all that we really know. It's just the one rule that checks out.
For instance, what about theft? Had you just come out of
Bicycle Thieves, you might suggest that it is wrong to steal because you don't know what role that that object has to play in a person's life. Had you just finished reading
Les Misérables, you might suggest that it is fine to steal if you need to do so in order to survive. From this, you could deduce that it is wrong to steal a bicycle, but not a loaf of bread, but doing so would be absurd. The percieved need to ascribe abstract truths to Ethics is due to a legal aporia which assumes that there ought to be an effective functioning of the State. A governing body has no idea as to how to deal with criminality without the disjointed logic of objective Ethics. To me, it is clear that Ethics are contingent upon the particulars of any given situation. Because we could never understand any given situation in its entirety, we can't make any objective claims as to what is Ethically valid aside from a few deductions such as that "murder is wrong".