It's a question that has been pored over . Earlier, Beric had expressed being tired of coming back from the dead: "Six times is too many." Perhaps he revived Catelyn because he was ready for a true death. Or perhaps he felt like he still owed the long-dead Eddard Stark something since he hadn't caught Ser Gregor Clegane. Or, perhaps he revived Catelyn to fulfill his promise to Arya that he'd reunite her with her mother. Either way, he gave Catelyn the "last kiss" and unleashed Lady Stoneheart.