The season six finale shows Barb with a slit throat and Carol crawling to her death with a knife in her back, all while holding a bloody shiv (presumably the one used to kill Barb). Some Reddit users have suggested that a third person stabs Carol in the back, and that could be revealed in season seven. It's possible that Frieda is the one who stabs Carol. (Another Reddit user suggested that Frieda kills both of the Denning sisters, which would be even more twisted.)
Image Source: JoJo Whilden / Netflix