Well, yes, but making Dean a helpless passenger in his own meatsuit while she tortures Sam would have been pretty awful for Dean as well.

I think that Dean really, really doesn't want to believe that Sam has gone bad, somehow, and that's why he didn't think that Sam wasn't himself. Although now that I type it out, that doesn't make much sense at all.

I think it's more that Dean will always always see every breath as an opportunity to save Sam, even if he *has* gone evil, so no situation is unsalvagable if they can only get on top of it and figure out what to do next. Hmm...