If they do not kill the character her role will have to be changed. I can not see the current status of Moria trying to undermine, kill Malcolm, why hiding it from her kids lasting too long. Also its going to be equally implausible for the Queens to become one big happy family when all this is revealed to Oliver and Thea. Not to mention the bigger issue of Oliver's own secret. If she survives the show will be radically different next year.