>I've read a ton of philosophy and something I don't really understand is that one nation killing another is more immoral than when a nation does this to their own domestic population.

Who holds this opinion?

>But for some reason, humanity doesn't seem to care as much.

All of humanity cares less about when a government uses violence against its citizens than wars?

How can you possibly make this generalization when each internal conflict is different just like every war and how difficult it is to measure sympathy