You already explained in your great post why we can't have moral rules like "killing is wrong"
Which is why I never use "killing" and "murder" interchangeably. Anyone who genuinely means "killing is wrong" is a nutjob if you ask me, for they fail to dinstinguish between aggression, defense, and punishment.
In its society, all moral judgments are made strictly based on a single good, which is the survival of the host organism.
That describes a selection heuristic, not a moral obligation. If a set of rules helps a society persist and outcompete others, then those rules may be effective strategies for survival, but effectiveness and obligation are not the same thing.
ruleset -> society survives or outcompetes rivals
Which is a true and
honest justifiable descriptive explanation for why certain rules have spread historically. And it does not yet explain why anyone ought to follow those rules. Anyone could consistently say "I understand that these rules help societies persist, but I do not care whether my society persists." Why would their choice be morally wrong rather than simply contrary to the group's interests?
An implicit "selector god" arises,
Renaming the survival heuristic doesn't change the structure of the explanation:
societies with certain rules survive -> those rules spread
So the same step remains: successful survival strategy -> ??? -> moral obligation.
As a general aside, many attempts to answer the central question of the thread essentially boil down to "these rules are good because they promote value xyz", be it survival, flourishing, God's favor, stability, and so on.
The problem is that every such value-based moral system is only binding to someone who already accepts that value. Anyone else can simply shrug and say "I don't care about that" and the argument stops right there.
That is, I don't think it's structurally possible for a value-based moral system to end up generating anything but claims that are conditional at the core, like "if you value xyz, then do abc". Which is perfectly coherent, but not the same thing as a categorical "you ought to do abc".
Now, the most common way that people try to avoid this problem is by claiming that the value is somehow objectively real (e.g. survival, flourishing, happiness, etc.) and then treating that as sufficient to generate an obligation. But as I keep saying, even if it were granted that such things are real features of the world, that still only establishes a fact about reality. The remaining step is explaining why that fact generates an obligation rather than merely describing a state of affairs. Without that step, the claim still reduces to "if you care about xyz, then you should do abc".
Another move which I have seen from another philosopher, thankfully not in this thread, is to argue that certain values are implicit in people's actions, meaning that if someone acts in certain ways they are already committed to valuing certain things. I say, at most, that shows that a person happens to act in ways that presuppose or promote some value. It would not explain why they would be morally obligated to continue doing so, or why abandoning that value would count as a moral failure rather than simply a change in priorities.