Out right unjustified murder is considered a taboo upon universal morals. There are universal morals, as evident by the code of Hammurabi, the Tang code, and Kouroukan Fouga. These are examples of written codes, doubtlessly moral taboos existed in societies without them where individuals exist, morals exist, where groups of individuals exist, societies form, and where these socities form the agreed upon morals are coded into law (though often with the king or warlord throwing in a few laws to justify or demand allegiance to the powers that be).
For instance, would you agree with the Hammurabi that theft, criminal negligence, assisting escaped slaves, disorderly conduct, multiple marriages, or being a 'bad wife' (the latter punishable by drowning) are justified killings. Why or why not?
Also, to say unjustified murder is a taboo is a truism - murder is by definition unjustified, hence why killing in self defence is not generally considered murder in our contemporary legal code. What matters is what is considered justified death. That differs significantly, and calls into question any real appeal to an authentic, universal, intrinsic moral-immoral narrative. It's determined entirely by context, and that being true, the only rational way to ultimately validate one's opinion is by force.
Keep in mind various species function differently, we don't see individuals within hives of ants (the workers at least) murdering each other at random.
Might doesn't mean right in my opinion, wars very often are meaningless struggles over where the line gets drawn on the map, perhaps it both parties were a bit more logical, they may have spent their efforts on improving their own civilizations.