@SpardasHeir and
@InedibleSpook
Ok so I was too lazy to replay the game atm XD so I watched the playthrough videos and exactly what Griffon said to V was "Gotta land the final blow, V! You know we can't kill demons!" and the "tips" also said Shadow can't finish off the demons and V has to be the one doing it. So there is a small chance Griffon did kill the mob character whose clothes V took, because that guy was human (maybe

).
The empusa, on the other hand, probably needs V to finish it? I just don't know if he could do it barehand or as he is now. As for taking the demon's energy, yeah V does take them after killing the demons, but apparently he needs the cane he always carries around to do so. I guess we have to wait until the next chapter to know for sure if Griffon could kill demons or not :/
Also SpardasHeir you should definitely play the game if possible, it's so good :3