How. If A is about to kill B, if B does not act in self-defense then B dies. So killing is not getting avoided.Of course it's arbitary, since the killing of any human can be avoided. That's not even talking about non-human life. So your moral system is just as subjective as the rest.
Yes, taking non-human life of higher animals capable of sentience is immoral. So an ethical diet would eschew animal flesh.