Sceleritas Fel was seen by Tav in Blood in Baldur's Gate, so it's safe to say he's not some hallucination, but that doesn't discount that he could still be an extension of the Durge's will, conscious or unconscious.
Tav saw something, we saw Sceleritas Fel. But was it really him? Depends on who was telling the story. What I mean is this: what if the fella could work like the Durge's separate personality, manifested to help him cope with the amount of killing and damage done by their own hands. Having a character with a DID could be interesting in terms of the narrative.
