I simply reject this notion of not being able to determine what is right and wrong. We CAN. Society is often wrong (slavery) but hopefully we grow and develop better standards. We live in a community, and what is right and wrong is best determined by what the helps the community survive and prosper. I can't prosper if I am killed...so I do others the favor of not killing them. I'd prefer to not be raped...so I don't rape others.
A golden rule approach to things usually isn't what's best for society, or at least this definitely is not always the case. For instance, you say we should do unto others as we would have done to ourselves, yet in a capitalist society, and really most societies, it is necessary to step on some toes to progress your status and possibly the status of the society as a whole; everything's a competition where we're all out to keep others done by raising ourselves up. Nothing can gained that wasn't denied to another, and no one wants to be denied what could help them live a better life.
Perhaps the case is that the golden rule is just an idea standing in the way of the truly powerful and clever meant to progress society. If If Qin Shi Huang had followed such an ethic, would there have been a unified China and their four great inventions for the rest of the world to take advantage of? Would it have been better not to have it? Was Manifest Destiny wrong? Would the world have been better with the natives unharmed by our diseases, swords, and gun powder, and the world been better without an America and all it's innovations?
You could also argue that such an ideal leads the whole society to be victimized if taken to it's logical extreme. Since no one could logically be punished for breaking the rule as no one wants to be punished even if they break the rules.
Among the standards we currently have is that a person is a sovereign being. Other nations may view people as subjects to a crown or slaves to a tyrant...and murderous tyrants are helped by folks like you who argue that maybe the tyrant DOES have a right to rape and murder children (I guess you'd argue that a child has the right to defend him/herself from the rape and murder...)...but in our society, we've decided that a person has certain rights which can not be separated from them. When those rights are separated from them, it is not right.
I don't recall of any tyrant in history who used nihilism as a justification for their carnage. I would expect nihilism to prevent them from justifying anything they do before anything else, and it's usually because they do believe themselves to be objectively moral that they carry out their acts of evil. I doubt King Henry would divorce Queen Catherine so eagerly if not for thoughts that made him think he'd never get a heir due to biblical immorality. Nor would he so soon kill Anne Boleyn for similar fears that he had sinned.
But you've yet to say how those standards are proven to be correct. All you've said is that those are the standards decided by the society, but who's to say the society is right. Another society might say a completely barbaric set of principles is right. Are they wrong? Another society might say that another society is in the way of progress and rob them of their land? Would that be wrong even if the alternative meant stagnation? Is it right for a group of invaders to take someone elses land if the outcome is better for the majority in the long run? Is it also okay for a society to attack another society out of pity for the suffering of it's citizens; don't those people get individual rights as well?
This is part of why I don't believe moral values can be determined. Who's to say what's to be valued as good when one man's moral system is nothing but a hinderance to another man or even society as a whole, and vice versa? It's all really a matter of perspective. Which in this case everyone will merely do what they think is right. Thus everyone is right and has the right to do as they able because everyone is wrong and cannot validate their view of things over the next persons.
