Talking about "validity" just seems to be a way to disparage any morality/theory/set of intuitions that's not your own.

You can promote metaethical objectivism without having and particular first order moral theory in mind; and you can hold that the Meaning Theory is a poor argument for subjectivism without holding objectivism to be true.

From a general level, anything that fills the cognitive role we talked about as a definition, assigning things something like blameworthiness, counts.

Not equally. Not without some hefty question begging. Anything that assigns solutions to numeric problems could be called arithmetic, but some assignments are true and others false.

and yes, that means the serial-killer morality too.

Counts as correct?

The way to avoid "dead-end relativism" - e.g. not stopping serial killers even though you think it's bad - is to be comfortable with being an agent with a morality the same way a carefully-built AI could be an agent with a morality. It doesn't actually matter that your morality could have been something else. It is what it is, and so it's true that when I say "right" I'm referring to Manfred::right, some specific algorithm, and I'll still stop serial killers because it's the right thing to do.

Unless you are one.

I don't find it satisfactory to be compelled to stop things--to treat them as if they are wrong--without knowing why, or even that, they are wrong. I like reasons. i guess you could call me a rationalist.

We're back to trouble with words again. Like the tree falling in the forest making a sound, "right" can mean different things to different people,

I've just argued against that. This is going in circles.

and the way to solve the problem is not to argue over who's "right" is right, but use more words to just care about the actual state of the universe. So I'll stop a serial killer, but I won't argue with him about whether what he's doing is right.

I think a universe where force is minimised in favour of persuasion is preferable.

But if it the argument goes nowhere, I'm comfortable with him doing Killer::right, and me doing Manfred::right, and then I'll hit him with a big stick.

What if you are really wrong? What if you are the guy who is rounding the slave owners "property" and dutifully returning them to him?