The artist guy has something locked up in the same area where you post the photos he makes you take. Do you get that if you kill him? Or is there something else you need to do? I remember him saying something like if you were his true desciple then he would show you.

Does anyone know what is in there and how you get it?

Yeah you kill him then loot the key off his corpse. Can't remember what was in it but it was nothing worth remembering tbh.