Blood+

[Ryllharu]

I think Saya and Diva are more than just sisters, I think they are twins. It looked a lot like Saya, but something seemed...off. That's why I think it was Diva rather than Saya. The haircut seemed closer to the one Diva had as she was laying on the ground in the Chevalier Atrium than Saya's classic straight hair (the long versions from the film and Vietnam).

That image was also from after the fire, which occured in the flashback the old man was talking about, but yes, it seems likely that either Diva or her servants killed off everyone, either that or Saya went berserk when she was turned/awakened.

The next eps should answer that question definitively.