Murder is wrong and the Earth is round.
The difference between "the earth is round" and "ice cream is good" is one is a statement of fact, the other is a judgement. Statements of fact like "the earth is round" are more or less tautological (assuming they are true)- "roundness" is a feature of our concept of "earth." So if we ask "Why is the earth round?" the answer is "because it's the earth." That it is round is a given of it being the earth. However, saying "ice cream is good" is not subjective if you add the hypothetical "if we are talking about taste" and the qualifier "to me." "If we are talking about taste, ice cream is good to me" is an objective truth (unless I am lying)- who could argue with what I say I enjoy? But there is nothing in the concept of "ice cream" that makes it so. I must apply a subjective judgement to make this statement true.
Now look at it this way. We are discussing the nutritional benefits of certain foods, and I say "Ice cream is good to me." This is not an objective truth, because I am not making a judgement. "Unhealthy" is included in the concept of ice cream, so there is no judgement to be made. Here we add the hypothetical "if I want to be nutritious." The statement becomes "If I want to be nutritious, ice cream is not good for me." This is true because the feature "unhealthy" is included in the larger concept "ice cream."
As this applies to murder, there is a tautological objective truth. Murder is loosely defined as "an immoral killing." "Immoral" is a feature of the concept of "murder." So the answer to the question "Why is murder immoral?" is simply "Because it is murder." However, the question of "Is this specific killing immoral?" (i.e., is it murder) requires a possibly subjective judgement, which also requires a hypothetical. "If we are interested in maintaining society," is a possible hypothetical here. "If we are interested in maintaining society, then that specific killing is a murder" does require a subjective judgement, but comes very close to a sort of objective truth. It does leave us open to some grey areas (like capital punishment and abortion), but in my opinion it's as close as we can come to an objective statement on why killing is wrong (Outside of the concept of the sanctity of life, where we could go on endlessly on what life is more sanctified than another in terms of capital punishment and abortion).
We do demand a justification for the conclusion that the world is round, which is that when we look at, it is in fact round. Believing it to be round is not a justification. We don't demand more of moral judgements. In both instances, we want there to be some justification. We might say that murder is wrong because to allow murder would make society less happy than if we prohibited it. That would be an objective basis for concluding it.
Of course the issue is not quite so simple. We don't, in fact, determine that murder is wrong because we find that it fails under Utilitarian principles (or any theory for that matter). We first itemize what we intuitively know to be immoral, and then in hindsight we construct a theory that attempts to explain why those items are on our list. Should the theory place something on the list that we intuitively know should not be there (e.g. we should kill an innocent person to calm our irrational society), we then tinker with our theory or disregard it altogether. This means that our justification is really nothing more than an attempt at an explanation for our intuition.
Anyhow, this may be beside the point. The question remains whether the wrongness of murder says something about the world at large or whether it simply says something about particular humans and their disdain for murder. If I say that murder is right and you say it is wrong, are we disagreeing, or are we simply telling one another whether it is right or wrong to us (much like ice cream may be good to you, but not to me)?
Well, if we determine whether the world is round by looking at it, and that is adequate, don't we determine whether we or others feel murder is wrong in a similarly non-subjective manner, by "feeling" as we do and ascertaining what others think by asking them or observing their reactions and conduct? We don't as far as I know demand that, once we have looked at the world and have seen it is round, we further establish that the fact we have looked at it is sufficient justification for our claim that it is round. Nevertheless, we require justification for the claim that murder is wrong above and beyond the fact that we are horrified by it. Horror is not enough in the one case; looking is enough in the other. I would say the wrongness of murder does say something about "the world at large" because we humans are very much a part of the world, and what we are considering is human conduct. I understand of course there may be those who maintain that murder is just fine, and there are those who commit murder, but might this be accounted for by the fact that the data under consideration (humans) are complex, and the inferences or conclusions to be drawn are thus necessarily less precise than in other cases? Most humans deplore murder. Most worlds are round. Why should most humans deplore murder? Why should most worlds be round? It seems we insist on asking the first question, but not the second.
Evidently we treat our own desires and conduct as special cases. Do we do so because we continue to believe that we are somehow apart from everything else, not subject to the same rules governing other parts of the universe?
My conclusion is that ice cream makes morality very sticky.