At one point in the history of man did it become considered wrong or taboo to kill other people?
When did people start punishing others for murder?
This is a tough one to answer, as you’ve asked it, because the question is too open-ended. However, most existing cultural records have some sort of penalty for murder – the Bible, the Code of Hammurabi, etc have penalties listed. They may not be as far as the current level – they may not include all people (foreigners may be fair game in that system), nor all stations (slaves may be excluded), etc. On the other hand, that really means nothing – currently some governments will still execute a criminal, so the degrees I mention above may not mean as much, if you look at contemporary society critically.
Most animals have some sort of instinct against random killing of their own kind. Of course, they compete with each other, and fight over territory and resources, or even wage warfare and predate their own, for example, chimpanzees in the wild. But such behavior has to be self-limiting, or that species will extinct itself. Prehistoric humans should have worked under similar controls.
Maybe you want to ask a more limited question that gets to the heart of what you’re really wondering?
No. It has always been wrong in the historical period, with many exceptions (war, capital punishment, honour killings. etc.).
Prehistory.
I can’t give a specific time but i suspect that it was right after someone realized “If Og kill Urk, Og kill me next”
I think it predates Homo sapiens, depending on what we mean by “wrong”.
We’re a social species evolved from social species. It’s likely we’ve always had an instinctive revulsion to harming members of our group, as a generalization. And it’s likely groups have always ostracized individuals that committed such actions.
When mankind explicitly thought of it as morally wrong though, or when/how we’ve tried to extend such instincts so that they apply to any victim (not just to members of our group) are different questions.
It’s wrong? Tell that to the POTUS; seems to be the one bright spot in his presidency. I shutter to think where his approval rating would be without killing people. 
One might even expect that natural selection would work against people in societies with too low a taboo against killing.
Honest question to the OP: Do you really think there is a known, factual answer to this question?
An even harder one is to simply ask when humans became aware of the idea that anything could be “wrong”. As above, most species will avoid killing their own kind. It is clearly of benefit to the species if this is a widespread trait. But moving from a behaviour to something that is coupled to a sense of morality - that is hard. One might guess that development of language was closely correlated with this.
Except that our closest relatives, chimps, routinely kill their own kind. Generally, it is a chimp from another troop that gets killed, but not always.
The earliest historically documented instance is the case of Cain and Abel.
So, much the same as humans really. Humans nevertheless think killing is wrong in most circumstances, including many of those circumstances where it does sometimes occur. Chimps may well feel similarly (even if they do not reflect on, and generalize about, these feelings as humans do).
When people realized that even ketchup doesn’t kill the aftertaste.
What’s the evidence for this?
That’s not how natural selection works.
Someone else mentioned chimps, so a good factual answer for OP would be to list which primates murder, and what is known of their taboos (or is “social instinct/behavior” the correct word?)
Cite? I wish Diogenes were here to confirm it, but IIRC the Cain-Abel myth might derive from an earlier Egyptian and/or Mesopotamian myth. Yes, such myth would not be allegedly dated before 4000 BC, but if it’s just the early alleged date you want, don’t some of the Indo-Iranian religions have (fantastic chronology) dates much earlier than 4000 BC ?
I meant it in the general sort of way. That is, in some way citationless. Trying to tease out of the OP’s own, more definite question.
For example, animals compete for food, and some die, as a result. But that’s not premeditated murder. Animals fight for mates, but at least in some cases, these are ritualized stare-ing, shoving, display matches. So you can see a precedent to avoid species destruction. This is an anecdote, not evidence.
There are many social animals, wolves and chimps for example. They defend their own group, wage war against other groups, and are prone to occasional internal conflicts. We don’t stop calling them social animals because of internal conflicts.
Lets consider Neanderthals. We have evidence that their corpses are surrounded by flower pollen. I can conclude that they buried their dead. You can call me a fool, because that is simply an anecdote, and not evidence. But that’s all I’m offering. Other Neanderthal skulls show definite human bashing in of the skull, presumably to devour the recently dead’s brains. This is also an anecdote. We do not know what Neandertal policy towards its own species was. The last of them may have extincted themselves in an orgy of brain-bashing-snacking.
These are all just stories I heard. Citationless though they are, they are a good starting point for the OP’s very broad question.
I assume the OP will return at some point. Probably to testify religiously.
I doubt that there is a factual answer, but I’d suspect that keeping someone with some of your genes from reproducing would be disadvantageous in the selfish gene sense. Anyone in your tribal group will likely share some of your genes, as opposed to someone in another tribal group, which could explain why in-tribe murder is wrong while outside the tribe murder is fine. I suspect this would kick in long before anyone reasoned about the consequences.
Cain / Abel is, as you have stated, a myth (in the technical sense of sacred narrative) and should not be understood as history. It does not derive from a specific account, though it will be related to similar stories in other ancient Semitic mythologies.
No, that simply isn’t true.
You might be able to make a claim that most species of mammalian herbivores will avoid killing their own kind, but even there I would want to see some evidence before I would believe it.
Once you start talking about omnivores and herbivores, the claim isn’t in any way true at all. Most species actively seek to kill and eat members of their own species whenever they can. Do you really think that spiders, crabs and crocodiles avoid killing their own kind?
Even when just dealing with mammals, the statement isn’t in any way true. Pretty much any mammalian omnivore or carnivore will actively seek to kill members of its own species if it can do so with limited risks. This usually presents itself in the form of eating any unrelated juveniles they come across. In many species, such as tigers or brown bears, the single largest cause of juvenile mortality is intraspecies aggression.
Of course animals will avoid attacking adult members of their own species, but there is no evidence I am aware of that this avoidance is any more pronounced than the avoidance of equally sized individuals of other species. Foxes, for example, are no more likely to avoid killing other adult foxes than they are to avoid killing adult cats or adult raccoons. And they are no more likely to attempt to kill unprotected kittens or raccoon cubs than they are to kill unprotected fox cubs.
In short, there is simply no truth whatsoever in any claim that most species will avoid killing their own kind.
Agreed, there’s not likely any altruism encoded in the genome of moderate omnivore and carnivore mammals. Just a general avoidance of conflict with something that can just as likely, harm the potential aggressor. Even so, we’ve already discussed the intra- and inter troop predation the Jane Goodall saw in chimps. So we know, its hardly a perfect scenario. When I started with Bible stories, and jumped to contemporary human societies, I skipped lots of stuff.
For example, Ancient Rome maintained a civilization, based on written law. So we can say the OP’s premise was well developed by then. And yes they had a system of justice, a collection of laws … and gladiatorial fights to the death of enslaved humans. So maybe we weren’t there yet.
Is the contemporary US system excellently conforming the OP’s premise? Last I checked, we also have a justice system – one that I’m willing to take a gamble with should the situation arise. One that nonetheless, executes criminals and targets terrorists, as Si Amigo: took the time point out. So maybe we’re not where the OP said we were.
Until the OP gives us some better definitions of what the question is. We might as well just spout random good guy bad guy anecdotes.