When I had first heard the story of Silas, and his lover I thought that she could have been a Petrova Doppelganger. I also think that Silas isn't really suicidal and that he had influensed everything leading up to Katherine being force fed the cure. Think about it he never made a move against Stefan until after Katherine was cured. Also that he could possibly turn her into a vampire descended from him, truly immortal. And when you think about it, it sort of makes sence that a Petrova Doppelganger and a Salvatore Doppelganger are soul mates, Katherine only ever loved Stefan, Elena was in love with Stefan, and Stefan loved both of them (he only got over Katherine after he met Elena). And if this is true it can easly be explained, when Silas became truly immortal and nature needed to find a balence Stefan was made, so a Petrova being his true part of her can never die so the line of Petrova Doppelgangers was made too.