naaaaah, I think it doesn't matter Matt has the knive, he can't use it on elena or she'll die. Don't forget: Matt was wearing his resurection ring whent he was stabbed.... I think. So he lives, but elena wouldn't be so luckey :/ . I hope I got this right xd

Actually Matt only survived because of the Gilbert Ring, and im pretty sure Elena will survive as shes a vampire, who can heal, and is immortal so...

I thought Matt survived because it wasn't a fatal wound, I don't think it had anything to do with the Gilbert ring.

181: I guess she gave him the knife before it happened that Katherine was dying - possibly also as a measure of protection because at the time Matt was the host for her boyfriend and to account for the potential of Katherine ever possessing someone. I think being stabbed with the knife does kill the passenger, not the host - hence why Nadia was so mad at Katherine for killing him. Also remember Katherine's suicide letter - she apologized for killing him. 

(Gregor was the boyfriend, by the way, Trevor was Rose's friend and died in 2x08)

I figured Matt survived because it didn't seem too deep of a wound (relying on memory here) and anyway, someone could've healed him with vampire blood off-screen. If Elena were stabbed with the knife, I suppose Katherine would be completely dead and Elena would probably heal instantly.

In the end of 'Fifty Shades Of Grayson' Nadia waits for Matt at the grill and she is devastated after a conversation where she thought Kat was seeking her forgiveness but was actually talking about Stefan. She asks him to keep the knife safe should Kat decide to do the jump into another body as she would need it after that to protect herself. She reminds him of the ménage a trois he had with her and Rebekah while touring the highlife through Europe (which we have had little detail of sadly). She appeals to the son who has personal experience of a neglectful and unwittingly bad parent and he gives in and says he will keep the knife. I assume he hides it somewhere on the lockwood house/estate as that's where he seems to be living. (haven't really explained what's going on there he says he legally owns the lockwood house but isn't paying the bills did he sell his house? Does he still have it and how is he paying for it? If he sold it and is living at Tyler's rent free that's going to be a big financial break for him but surely his mum is on the deed for the Donovan house and he can't sell without her.) He survived the would because of the ring the knife and passenger were supernatural and kat stabbed him who is still a doppelganger even if human at this point. Elena/her body are vampire so her passenger would die and she would heal. It was a small dagger but it was to the hilt I think she stabbed him in the lower left torso so would have been potentially lethal had he not been protected.