Because you have to draw the line somewhere.

Morale/ethics do change over time, but not killing people has been shown throughout history to be the bare minimum for a decent society, and an accepted standard today. We can discuss and expand on adding more species, kingdoms and domains, but humans are not up for discussion.

    > to be the bare minimum
    > for a decent society

So if a utilitarian-leaning society could show it was improved by killing people, you'd be fine with that?

I would not be fine with that, no, but then, I'm not fine either with killing other apes and other self-reflecting animals ; my standard vs modern society's accepted standard.

However, in that hypothetical case you describe, it might be up for discussion. My point was that by modern standards, killing other humans is not ok, and not up for discussion.