New York rules its cool to look at kiddie porn

Kill over ridiculous reasons? They do.
Ok. This might seem like a copout, but I am so used to defending a market system, that I am happy these days when we can accept that some things in the human condition that we have under statism will continue under freedom.

Is subjectivism something like: you believe in something and so to you that belief is true?
It's not so much a belief, as a preference.

If I like chocolate ice cream more than vanilla, and you like vanilla more than chocolate, there really is no way to say one of us is right and the other is wrong, because the answers are intrapersonal (our own).

This is why some (honest) economists will say that it's not possible to do interpersonal value comparisons, because there is no way to compare your love for vanilla to my love for chocolate except as conclusions.

We can't even analyze that you and I are using the same cognitive processes to come to those conclusions.

They misunderstood the facts though. Based on reason (which is based on evidence), they did not act rationally. Again this assumes that some sort of absolute morality/reason/rationality exists.
We know that no absolute exists unless we have perfect information.

Because they worked from a flawed information set, they made the rational conclusion relative to what they understood.

You've gotta understand that your conception of perfect rationality means that everyone is wrong all of the time, because it presumes we all know, and understand every fact.

Obviously, that is not, and may never be the case.

How can you know something without evidence?
Deduction.

Deductive reasoning - Wikipedia, the free encyclopedia

We would need to know what absolute values are, yes. We're obviously not at the point of perceiving them now, but you could argue that we're evolving towards that knowledge. Just because we don't know what objective values are doesn't mean they don't exist.
In order to know absolute values, you would have to know everything. You'd also have to exclude the notion on a human soul.

If everyone knew everything, reality would be fundamentally transformed. There would be no arbitrage, because everyone would value everything the same, eliminating any function for trade.

This is true. I think it's conceivable that at a certain point (if we make it that far) humans will be able to understand absolute values. Many of us understand absolute values today (ie killing innocent people is wrong). The trick then will be acting only on those values. It's not following a script, it's evolving towards more prosperity.
Killing innocent people isn't an absolute value though, it is a subjective personal moral value. You can't prove that killing innocent people is wrong. A moral statement is an OUGHT (you ought to do this, you ought not to do that) and a fact 2 + 2 = 4 would be an IS.

They aren't the same realms of knowledge. They are different domains with different rules and characteristics.

I can see where you're going with this, because it does seem slightly absurd to conceive of a way to measure whether or not chocolate or vanilla is better (based on how your body chemistry reacts to the flavors). It would be rather tough to argue with someone and tell them their brain is tricking them into believing chocolate is better than vanilla.

In the same way it's tough to argue with someone and tell them their brain is tricking them into believing they talk to God when they pray.
Bingo, which is why you will never see me criticize people's religious beliefs.