100% nurture you say, that's mighty bold of you. To clear my point up, I don't believe as children we're born with philosophical ideas.
again, this is down to presuppositions, are morals completely made up?
I personally don't think so; the brain is a complex thing, we barely know anything about it, and because of this I think it's logical to assume that there is a sense, deep within us (rooted in psychological factors, evolved and adapted to a radically changing world). It certainly would explain our inclinations to act in certain ways, esspecially if you think that children don't alays turn out like their parents.
Now, our reasoning is something completely different, we can choose not to listen to these 'morals' or inclinations based on other factors. For example we have a powerful urge to eat in order to survive when ever we get hungry, but the thought of being thinner is sometimes the over riding influence on our actions, and so we refuse to eat.
Murder I think is wrong to all, but put into different contexts, human sacrifice for your God, revenge for a wrong doing, punishment to maintain order within society could all be reasonable factors to over ride this initial thought that murder is wrong.
The fact that 'special circumstances' have to be found to justify murder, suggests to me an internal or external influence. Sinse I don't believe in God, and the theory of evolution makes a lot of sense to me, I have adapted my own view to suit me.