So there is this article/post which reports of Jensen saying at a convention that his reaction to reading the season finale scripts was the following:
Well, let me say this. I got to the last script, and just went ‘Oh. Wow. Oh wow. They went there.’ And that’s not something I often say.
I want to hope for Destiel, I really do. But I don’t think this is what’s going to happen because – it’s the season finale of a show that is about two brothers fighting the supernatural. Plain and simple. So if they want a huge season finale (cliffhanger) that appeals to everyone of their audience and not just the shippers, they will go for something that is related to the show and its basic plot in general. A Destiel kiss/confirmation/whatever would make many people happy (me included), but leave the average viewer out there who is just watching and not a fan per se confused.
Therefore, with what Jensen is saying, I assume that they’ll actually go with what has been alluded to by the whole MoC story – they’re going to have Cain kill Able – meaning, Dean kill Sam, or at least try to kill him. Maybe it ends with a situation in which is not clear whether Sam will really die, or maybe they go through with it and next season has the brothers apart, but Sam still sort of hunting from heaven. Maybe they turn him into an angel, lol ^^
It was said before that they’re going to do something they’ve never done before. Sure they have killed Sam before (sort of), but he has never stayed dead. If the conclusion to all of this is that Sam and Dean are Able and Cain, that this is their destiny, who is to say that after nine seasons, not knowing where the show is heading (there could be an eleventh season, or they could be cancelled after the tenth, who knows), they really do go through with it and kill Sam? It IS Supernatural, after all – killing him doesn’t mean he won’t be part of the show anymore, even if he stays “dead”.
Personally, I’d love for them to do that, because I think it holds endless opportunities as to where to go with the show. Dean and Sam apart? Hear my heart breaking, but I think I’m still up for the challenge (of seeing that ;)).