By that logic, you could justify absolutely anything.
Our standards have changed, too -- not even a century ago, holocausts were considered absolutely appropriate (even by scientists!).
Plenty of societies, both ancient and modern, have legalized, or even encouraged, what we define as 'murder'.
And note that I'm not arguing that murder is acceptable; rather, that the definition for it changes so much between societies as to make the statement "Murder is wrong in all human societies." a completely meaningless phrase.
A likely explanation for the prohibition against the killing of other humans is strictly pragmatic: Societies where members can kill each other freely will self-destruct. This has nothing to do with 'murder being wrong', as much as free homicide not being a very stable evolutionary path.