I'm with you, I think most likely Dean won't KILL Cas but will attempt to while under the control of the Mark, and will end up only rendering him mortal, permanently.

I really hope Crowley doesn't end up dead. :( But I'm afraid it's starting to look that way. :(

Only thing is, there's nobody to step into his place as like, the epic, awesome antagonist. Rowena certainly doesn't have the appeal that he has, and definitely not Metatron. So maybe they'll let him live a while longer.

But narratively speaking, it only makes sense that Dean will "kill" both Cas and Crowley in some form before the end of the season. :-S