Long before anyone else realises that Daenerys is heading down a dangerous path, Sansa knows that the benevolent Dragon Queen is hiding a darker side. The Lady of Winterfell has been two steps ahead of everyone else all season long, and by sharing Jon's paternity secret with Tyrion, she ensures that Daenerys's reign begins to unravel before it starts. While it's hard to imagine Sansa personally killing the Mother of Dragons — she's not exactly known for her swordplay — she could be the one who sentences the Targaryen Queen to death.