How is this different from, say, the external world?  Like, in both cases you'll ultimately ground out at intuitions, but nonetheless, the beliefs seem justified. 

Moral realism is just the idea that some moral propositions are objectively true, not that all of them are true. 

They're doing nothing subjectively wrong if they really don't know.  But if they knowingly don't look into it then they're a bit blameworthy. 

There's a distinction between subjective rightness and objective rightness (these are poor terms given that they're both compatible with using moral realism).  I'd say that if you torture someone thinking it will be bad but it turns out good, that was subjectively bad but objectively good.  Given what you knew at the time you shouldn't have done it but it was ultimately for the bets.

See my reply :)

Objective just means that its truth doesn't depend on what people think about it.  The Earth being round is objective--even if everyone thought it was flat, it wouldn't be. 

//I think that these things really are wrong and don't depend on what people think about it. But I also think that that statement is part of a language game dictated by complex norms and expectations.// 

To me this sounds a bit like moral naturalism.  You don't think morality is something non-physical and spooky but you think there are real moral facts and these don't depend on our attitudes.  

I guess I don't quite see what your puzzlement is with morality.  There are moral norms which govern what people should do.  Now, you might deny there in fact are such things, but I don't see what's so mysterious.  

Richard Chappell had a nice post about the last kind of objection https://www.philosophyetc.net/2021/10/ruling-out-helium-maximizing.html

I also wrote something about this a while ago https://benthams.substack.com/p/contra-bush-on-moral-fetishism?utm_source=publication-search

Morality is Objective

Um, see above :)

Biodiversity isn't ultimately what matters but unfortunately it's the best proxy that we have for learning about the distant past.  There aren't really studies about past NPP after mass extinctions.  More diverse ecosystems tend to be richer and more productive. 

Also, humans have, in fact, been drastically reducing insect populations--https://reducing-suffering.org/humanitys-net-impact-on-wild-animal-suffering/

Why does being cage free need more feed?