(
Melanie was always pushed as the one who initially stabbed Stella - but I keep going back to Melanie finding Stella in the tower room and the scenes that followed and if Melanie had stabbed Stella, wouldn't Stella have reacted to her differently?
I've always wondered if it had been Justin who did the initial stabbing? But I guess we'll never really know for sure who it was...
And as for who killed Stella, well, there's almost a whole houseful of suspects if Stella threatened to reveal who stabbed her, in which case she could have been killed to silence her. But again, I guess we'll never really know for sure who it was...)