Shadowbird: I understand a person arguing that morality is relative. The existence of moral relativism is widely documented and, in our civilization, quite popular. It's your prerogative to claim that moral absolutism is false. What I don't understand is your claim that the concept of moral absolutism doesn't exist.
The existence of moral absolutism is also widely documented. The Abramic religions are a convenient example. Huge portions of the Old Testament consist entirely of variously specific enumerations of Things That Are Evil. Worshipping idols is evil. Making burnt offerings is good. Breaking the sabbath is evil. Loving Thy Neighbor is good. Doing what God says to is good and doing what God says not to is evil. I don't think anyone here is likely to deny that there have been people who use similar concepts as their moral compass. If that's not moral absolutism, I don't know what is.
The moral relativist here might say that "This is just one kind of moral absolutism, what about the ones who say that idolatry is awesome and the sabbath is for suckers?" but he would be missing the point. The moral absolutist does not say "My God proclaims this to be evil, and your God proclaims it good." He says "This is evil; your God is wrong to proclaim it good."
The fundamental difference between moral relativism and moral absolutism is this: under absolutist morality, the good and evil are facts existing independently of belief, and any moral judgment of others is made according to these facts; under relativist morality, good and evil are beliefs held at a less fundamental level than truth, and moral judgment of others is either based on the other's beliefs or not made at all. What specifically is believed, the results of these judgments (i.e. punish, forgive, ignore, banish, chastize, reward, etc.), are details specific to individual moral models which can be categorized as either absolutist or relativist.
Moral absolutism, typically, emerges from homogeneous cultures where change and the prospect of change are both very minimal, and neighboring cultures are either very similar or very hostile. In such circumstances, societal stability comes mainly from not rocking the boat; an unambiguous moral model forms the best basis for the ethical and legal framework of that civilization. The rules are finely tuned for the situation in which they arose, and deviance from them is suboptimal.
Moral relativism, on the other hand, emerges from diverse cultures in dynamic times, where there are many exotic neighboring cultures, most of whom are trading partners. The modern West is an excellent example. In these circumstances, stability and solidarity comes from coexistence and acceptance, since the boat is already being rocked; a moral model which allows other moral models to coexist underneath it will produce a legal and ethical framework that keeps that civilization running smoothly and profitably. There are lots of people who'd prefer to carry around their own cultures than learn a new one, and it's far preferable if everyone agrees to let everyone just do their own thing. People who form their own moral models in such an environment will base it on the environment's highest prerogative: don't pick a favorite.
It's basically the difference between Right And Wrong and Live And Let Live. Adhering to one of these philosophies does not grant one the ability to proclaim that one is merely a creative interpretation of the other.
I suppose I should tie this back into my main point.
People exist who adhere to absolutist moral models. For this reason, it is entirely valid to make a game with an absolutist moral model. Simply design the game world's moral model after that of a moral absolutist, rather than a moral relativist. Done well, that could be a quite meritorious exploration of morality.
A game of this type is a type of simulation. A simulation is a system which assumes that certain models are accurate. Since moral absolutism is a moral model, there's no reason to suggest that a game can't be made from a morally absolutist perspective. That's what I meant.