I agree that Cora is going to die but I also think that something else we haven’t thought of will happen. We assume Cora dies and Rumple lives and retains (though there is strong debate about this) the Dark One’s curse, but I think something else has to happen to Snow. She’s going to go Dark, but does she go as Dark as Cora?
I honestly don’t expect Snow to go completely 180 and become evil. She is acting purely to protect her family and friends. I think she’ll even show some remorse for her actions, judging on her reaction from the promo. However, she won’t completely be the goody two shoes pushover she previously was.