Is it possible that an objective standard exists even if someone is not using that standard?
Possible? I suppose it's "possible", just like most other things.
Let's say that there's an officially standardized yard stick (exactly .09144 meters) sitting on my workbench. If I used three lengths of my own feet to measure out a yard, would the stick on the workbench vanish into non-existence or would it still be there ready to be used?
That question answers itself.
Sure, but that's a physical object that's used in a quantifiable manner, which is entirely unlike what we're talking about here.
Likewise, the fact that societies throughout history have played fast and loose with their morals, including their tolerance for murder, is not evidence against the existence of an objective standard.
Not by itself, no. The evidence against this objective standard is that.....well, we don't see any evidence for one.
The standard exists, the fact that you either don't know what it is or willfully choose not to use it is not relevant to the fact of it's existence.
Then where is it?
Let me ask you a question...
You say that whether murder is right or wrong depends on the subjective standards of a society.
No, I specifically said "murder" is always wrong in every society, because that's exactly what the term "murder" was created for. So it is whether an act of
killing someone is right or wrong depends on the standards of the society in which it occurs.
Would you say the same about every other right? If people do not have an objective right to life, do they likewise have no objective right to defend themselves against attackers or to own private property?
Of course. Even the Bible shows that to be true. Look at the examples of God ordering the Israelites to kill everyone in a town (except for the young girls they found attractive) and taking their possessions. Thus, according to the Bible whether or not it's ok to take someone else's possessions (or even their kids) is relative to the circumstances at hand.