Keira becomes increasingly short with you, ending the conversation abruptly, perhaps with a few truths that hit a little close to home. At any point during the conversation, insist that she hand the notes she made on the plague over to you. She isn’t about to part with them, and the argument degenerates into a fight. Slay Keira Metz (beware of her powerful shield and magical lightning strikes; you may need to retreat and use the tower for cover, as well as dodging constantly), picking up and burning the notes from her corpse as the quest concludes.
As you might have guessed, Keira Metz is not available to help defend Kaer Morhen during Act II: Main Quest: The Battle of Kaer Morhen.