Well, it's true that if too many murders take place then a society can't survive - in fact a good sign that a society is running out of resources is generally when it starts resorting to cannibalism (because they're short of protein). However at various times, we do condone killing other groups of people eg in times of war, for sacrifice. And we also make decisions about eg when to turn off the respirator. What we don't do is allow everyone to kill whoever they like indiscriminately.
Anyway, I do recommend Hrdy's book which covers both humans and other primates (LM, you do realise we are primates!). For example, she states that women are unique among primates that, like us, produce only one baby at a time, in that they actually kill their own babies. "When women cause someone else's death (through sins of omission as well as commission), that person is most likely to be her own newborn baby...Infanticide is hardly unique to humans, and it is widely documented among primates, both human and nonhuman. But in other primates [which produce only one baby], the killer is almost always an unrelated individual, never the mother." p. 179
"Almost all infanticide in traditional societies occurs right after birth, and is conceptually identical to late-stage abortion. Neonaticide is favoured over abortion becuase infanticide is safer for the mother. The situation is reversed for societies with Western medicine. Abortion - especially in the early stages of pregnancy - is safer for the mother than giving birth is." p 470
And this statistic from Papua New Guinea - for the Eipo, infanticide accounted for 430 out of a thousand infant deaths. However the death rate of the survivours - 50 per thousand - was far lower than in other developing nations. And this was a maternal decision, not forced on the mother by males in the society, as the anecdote about a mother who intended to leave her 3rd child (a girl) to die but decided to keep her (p. 455).
The point at which we decide a child has a right to life is arbitrary - conception, 24wks or birth or even later. Conception has no more necessary moral validity than birth. It is up to society to decide where that point is, and that will depend on lots of different factors, including, perhaps, the success of modern medicine in giving premature babies a good chance of a happy life, but also the costs associated with childbirth itself.
Finally, my point re eating cows is that we do decide that cows have less right to life than humans - but there's no obvious moral reason for that. We make these distinctions between species, but we also make them between different groups of humans. For some groups (humans and animals), we put a lower value of life. Unpleasant but true.