With the situation for season of the splicer coming up all the time let's explore the possibility to say Savathun is playing everyone.
What if Osiris isn't corrupted/hidden away/anything other than normal?
Let your theories fly. Mine is simple:
Savathun is a master of trickery. Perhaps she intended for us to focus on one guardian who responded to her song like Osiris and has instead been doing something behind our backs the entire time within the Vex network. Perhaps Mithrax was wrong about Quria being the center of the vex curse and she intends to plunge the last city into the vex network and reign chaos on people.
I think osiris is actually not the bad guy here, in past lore, osiris was kicked out for a reason. Osiris was the guy that always questioned the light and the way the speaker ran things, he was always seeking knowledge even if it meant doing shady things. The way he has been behaving lately is actually nothing new, this is what the real osiris is like. The guy is not afraid to seek answers even if it means doing things no one agrees with.