I think it rather silly to bring up a fairly rare example, and pretend we're talking norms. And in any case, war is generally recognized as something seemingly okay, as we've even gone to the effort -- through much of recorded history -- of establishing the rules by which we play that game. So, hard for me to wonder why someone who thinks killing in warfare is okay, but not to eat the resulting flesh. After all, say Flanders and Swan in "The Reluctant Cannibal" song, "if the JuJu had meant us not to eat people, he wouldn't have made us of meat!"
Now, just so you understand, I'm not a war buff, either. I don't like the notion of killing people just because I think I should own their land, or rule their people, or whatever.
However, if I'm not mistaken, war was generally okayed by the Bible, as well -- and often enough, even encouraged by God for the sole purpose of taking a people's territory away from them and making it your own. So this brings back to your original question (about whether something should be considered ethical until shown not to be): "Shown by whom?" And that establishes that you believe that it is necessary for some external "whom" to decide for you what you think is ethical. So I was wondering, since I don't believe in your God (or his wars to conquer Canaan), to whom should I go? Or do you think that, because I'm an atheist, I can, indeed, work it out for myself, even if some religious types can't?