What I am saying is that no process in nature demonstrates any form of ethics.
Well, that's not entirely true since animals still exhibit cooperative strategies, which is essentially what ethics boils down to, on an evolutionary view. But I still agree with and understand your basic point- that ethics and morality (as bodies of discourse, rather than simple behavioral dispositions) are human constructs. The sketchy part is what conclusions we want to draw from this- it doesn't follow then that, for instance, how animals behave can tell us anything about what our moral codes should look like.
How does killing a human interfere with our ability for conscious self-determination?
Their ability for conscious self-determination. And if someone is dead, clearly they aren't consciously determining their actions anymore.
Of course, because there are rules in societies to keep them functioning. It's not necessarily morality that makes this happen, its practicality.
I'm not talking about laws, but mores and values- which may often underlie or provide a basis for legislation, but are nevertheless distinct.
But other examples that have taken place in history would be a group of people wiping out another group of people because their land has better resources. The first group has apparently benefited from killing other humans because of the circumstances surrounding it. It's not as simple as the act of killing, it's the circumstances that surround the act.
So, why are certain circumstances excusable and others aren't?
I don't see how that example throws any sort of curveball for the evolutionary view that our moral sentiments are derived from inherited cooperative strategies; for instance, I'd imagine most people would say that, if the group killed the other group because THEIR land was barren of resources and they basically had to do this in order to survive, this would be grisly but not necessarily immoral. But the tricky part for the evolutionary account is saying how, exactly, the development of culture and society have affected the evolutionary trajectory of morality, if at all (it would seem it has).
And in biology, there is no such thing as ethics so, by that reasoning, killing a guy and raping his wife could bring benefit to your genetic line. That's why forced copulation is a viable mating strategy in the animal kingdom.
Sure- but it also could harm your genetic line since killing and rape often have repercussions, such as retribution from the other persons kin or society (or even your own). That's the point of morality from the evolutionary standpoint; it is an adaptive strategy intended to maximize reproductive success, in the face of
all the various environmental factors which may be in play (not just the obvious ones such as "I kill guy, I get his stuff")- and this variety of conditions likely explains why our moral intuitions often conflict.