I agree there is no. way. Dean can kill Sam. Not after all the events of the series so far. Dean's gone waaaaay out of his way, many times against his better judgment, to keep Sam alive. It would make absolutely no canonical sense if he killed him after everything they've been through together thus far. I honestly don't think Kripke would ever do that though. I mean, he likes making the fans squirm, but so far it's like both brothers have 9 lives. Somehow or other, they always make it back to each other. However, I personally wouldn't mind a Thelma & Louise style ending. SPN is dark enough that it might just be the only ending that makes sense (rather than, say, having the brothers drive off into the sunset in the end).