If I were telling them that they were not fully human because voices in my head told me they weren't?

I'd hope they would have the gumption to smack me upside my magic-thinking head.

Love your neighbor as yourself. From this comes all the laws and all the prophets.

That being said, yes, it is pretty disrespectful. I just don't see any reason to respect that belief -- in the same way I don't see any reason to respect the Klan beleif that black people are subhuman, either.