Anyhow, back to some sort of topic: You seem to be saying that Parfit is not claiming his theory as any sort of One True theory. Is this accurate?

Surely anyone who argues for a theory is saying that.

I dunno, you could just write down your theory to get it out there, maybe to convince other humans (which is possible, us being imperfect) as a means to spreading your morality.

the correct way to handle that theory is to say that different people have different theories/intuitions. Otherwise you fall into the trap of saying there are no real disagreements about morality, or that serial killer morality is perfectly valid because serial can make up their own meaning definition of "moral".

Talking about "validity" just seems to be a way to disparage any morality/theory/set of intuitions that's not your own. From a general level, anything that fills the cognitive role we talked about as a definition, assigning things something like blameworthiness, counts. And yes, that means the serial-killer morality too.

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.

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, 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. Well, I guess that's an oversimplification - humans are persuadable about the darndest things, so arguing about "right" is sometimes fruitful. 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.