There's something i don't get yet...
If Kat's inside Elena, then why would Nadia give the Traveller's knife to Matt?
It kills the passenger, right? If Elena/Kat gets stabbed by it, wouldn't it definitively kill Kat?
My guess is the knife really kills the owner of the body, and lets the passenger stay, which means that Matt's dead and the one inside him all this time has been Trevor, anyone else?