Why am I being attacked by the NPCs in the archive on the roof? (Dark Souls 3)

[aVm] Mors
So what? If they are non-burning, do you think they won't hit you?
These are the protectors of Lothric - the magician, blackhand and knight - the three pillars of protection.

Raibow Bitard
Aren't they all black-handed, just with different gear?

[ASASIN]
Blackhand with a pair of katanas and a bow.
Blackhand are sort of spies, agents. At least the ring implies that.
There are only 4 special professions in Lothric, each profession has its own ring: Knight - strength, Mage - intellect, Blackhand - agility and also like a Hunter.