Nihilism obtains. However, there is a behaviour that approaches morality in the limit, ethitropism.
Ethitropism + property recapitulates all the noncontroversial parts of morality, e.g. English Common Law, strongly suggesting it is the underlying principle and can be used to audit e.g. English Common Law.
Ethitropism is cooperation.
Morality cannot simply be might. If right can be reduced to might, then nihilism obtains. I require morality to alter behaviour patterns, not to be identical to any pattern.
Morality cannot simply be prudence. (Ethitropism is simply prudence.) If morality can be reduced to prudence, then nihilism obtains, because it is never necessary to sacrifice for morality; just the opposite, the immoral person is the one sacrificing. Were this so, intentional punishment (sub specie aeternalis) would be unnecessary, because 'immorality' would automatically punish. Pragmatically, it must sometimes be imprudent to be moral. Pragmatically, intentional punishment is necessary, so the assertion also looks empirically false. (Ethitropism does have long/short term tension.)
Not being merely accruing benefits, morality is also not merely avoiding costs; morality is not mere cowardice. If morality is cowardice, nihilism obtains.
I've spanned the space of all external reasons to alter behaviour.
This gives us some strong hints about where to look, which is inside. (It helps that many traditions already know this.) A good person is internally motivated to do good, not merely because that causes to happen more often, but because they do not stop being good if external conditions change. E.g. someone who truly was only good out of fear of jail or hell would stop being good if they enter an environment of anarchy or atheism. (Personally think the former deserves my friendship if they want it and the latter does not.)
For short, I call this internal motivation ethitropism and those that lack it anethitropic.
The moral and amoral, if your context is tolerant of imprecision. E.g. the anethitropic can still act moral if the surrounding society is ethitropic and thus converts, by intentional punishment, what is moral to what seems prudent.
Problem: helps not at all in defining 'good.' An ethitrope who is mistaken about what is good is as ethically useless as the anethitrope. However, good is valuable and important. The ethitrope, then, prefers to uphold the preferences of others. (Not instead of their own, but in addition to their own, as otherwise we have a contradiction where the ethitrope's defining preference is invalidated by ethitropism.) Hence the importance of the reification of values.
Bonus: anethitropes don't care what is and isn't a valid value, and will never be (directly) affected by these facts, which simplifies the analysis.
The principle that resolves conflicting preferences is property. For what you own, what you value goes.
There is one coherent definition of ownership, which is reasonable expectation of control. This is the minimum definition of property.
If I do not control my property, then I have a contradiction - someone or something else must control my property, thus by symmetry I must be able to control their property but not my own, including their body, through which they exert control over my property, which means I both control and don't control my property.
If I do not expect to control my property, I have a contradiction, as I will not attempt to control it and de facto control will go to someone or something else.
If my expectation is not reasonable, then the definition is infinitely abusable. Rationally, everyone would claim to own everything and we would have a contradiction. The mad frequently claim to control things they don't, and rationalize their failure to control it rather than learning.
In the limit of perfect communication, observation, and rationality, theft and so on is impossible. As time passes, our imperfect world approaches this perfection. If the customer steals from the ice cream merchant more than once, there will soon be no ice cream to steal, as the merchant will realize their expectation of control was not reasonable, and stop producing ice cream. The thief destroys their own livelihood in the long run. (Thus the long/short term tension first appears.) Given that the short term converges on the long term, I can perform the analysis purely on the long term.
Preferences regarding non-property cannot be realized (or else can be converted to property transactions). They cannot possibly be valid as values. Conversely, preferences regarding property will only not be realized if the preference-holder has higher preferences that supercede them.
The modus tollens is also important. If society considers a value valid, it must make or the corresponding property sufficiently secure, or it contradicts itself.
Ethitropism, then, is respecting the property of others. Since ownership is rarely absolute in the short term, it is always possible to find ways to attack property. The ethitrope does not attempt to do so.
It is thus cheap to cooperate with an ethitrope. Making contracts with them is always secure, because they cart around your security internally.
It is thus prudent to be an ethitrope. (Which means nihilism obtains - even internal motivation doesn't create a true ought.) Similarly, it is very rational to ostracize anethitropes. I give up minor gains in the present for the prospect of ongoing future gains.
Because property converged on ideal property, it is similarly prudent for society to promote and approximate ethitropy. Anethitropic societies will make their children poorer, not richer. (Sound familiar?)
It is very unwise for the ethitrope to uphold the values of anyone who will not symmetrically uphold the ethitrope's values in return. Gains from cooperation don't materialize if they won't cooperate.
Thus a full definition of ethitrope is one who upholds the values of those who will uphold their values. I have an amusingly recursive definition: an ethitrope upholds the property of other ethitropes.
The logical reason is that the property of anethitropes cannot reify their values, because their preferences point to violating values.
Why exactly is ice cream theft (very closely approximately) wrong? If the customer does not uphold the merchant's valuing of money, it proves the customer does not value value. The value reified, through the customer's ownership of self, is the anti-reification of values, causing all of the customer's other preferences to be ethically irrelevant, even to ethitropes. This is in addition to the fact that if the merchant knew the customer was planning to steal the ice cream, he either would have put in sufficient security to stop it, or simply not made it at all.
Alternatively, the merchant should steal the money as soon as they can, and never make any ice cream. Because the customer has no reified values, it does not violate ethitropy to do so, and there's no other way the merchant is going to get the cash. By deciding to deviate, the customer makes the prudent strategy pre-emptive annihilation. At best, the customer manages to fend off the merchant, but then the merchant will develop better weapons, and not only will the customer get no ice cream, an expensive arms race will ensue. The customer pragmatically destroys their own value, as a result of logically destroying it.
Conversely, the merchant can plan to kill the customer, and keep everything for themselves. But, by doing so, the merchant invalidates the valuation of their own life. It is not (approximately) wrong for the customer to kill the merchant in self-defence.
Fraud is of course merely intellectual (versus physical) aggression.
Ah yes, that terrible weasel-word, aggression. Using this framework, we can define aggression: it is abrogating or attempting to abrogate reasonable security.
Of course now we have a new weasel word, 'reasonable.' There is no way to define reasonable to a hostile audience. It is something for ethitropes to define by contract, which by definition they will always be able to do.