Yeah that is highly problematic for me. It's hard for me because I don't believe that. What's the point then? If Sam can't be changed or saved at all, then I almost feel like this is a waste of my time. I know it's a horror genre and horror stories usually don't end well, but at the same time stories of redemption and salvation are very archetypal. So I would think the show would be heading that direction and that the Castiel warnings are a mislead.
So I don't think it's true. if it is true, I will be ticked.

And if Castiel doesn't know what will happen (if he is telling the truth) then how can he urge Dean to kill Sam if that is what it takes? If he doesn't know Azazael's plans, how does he know what Sam's destiny is? There is a lot of gray area there to exploit.

Free will has to fact in here somewhere. If there isn't some kind of out for Sam, I will be really ticked.