I think Aborted touched on what I would say, I know there are tons of animals that will kill each other, selfdefense/defense of territory/mating rights.
It would be a hard discussion due to todays civilization I do not think we truly know what could be right or wrong because we are bred into believing that what the law says should be morals, which isnt the way to go about a discussion about what our base morals were when we truly had animalistic behavior
Now since animals have no true sociality to answer to they are in as pure a form as we were when we were apes -> cavemen and the like.
I cannot say I think killing is truly wrong. I don't think that with our civilized world today that it should be happening either, but thats what happens when our morals are based on law from enviroment.
Factors such as ability to communicate, which animals mostly cannot do as well. Levels of attachment to each others , which most animals only have within immidiate family, come into play if we were to try to bring the theory to human species.
But, also there are many animals that kill their mating partner after the deed is done (praying mantis) and kill their own offspring (dogs at times) and some that will kill their own brother/sister soon after birth (sharks).
So my theory has holes in it
