The book gives no single reason. Anna Rosa, Dida's friend, shoots Moscarda in her room, and the reason she herself swears to the judge is that there was no assault at all, only the fascination his reflections on life exerted on her, to the point of leading her into that madness. Moscarda also finds true the other reason she gave in her defense: the instinctive and sudden horror of the act to which she felt herself dragged by that fascination.
Anna Rosa had summoned him to the Abbey so that he would speak to the bishop before they had him committed, and in the corridor her revolver, a keepsake of her father's, goes off by accident and wounds her foot. She had already been tempted to try it against herself. When Moscarda, during her convalescence, explains that to know oneself is to die, that she spends so long before the mirror because she does not live, and that one cannot live before a mirror, she listens with attraction and with repugnance.
In the end, she stretches out her arms; he falls from the bed, wounded in the chest by the small revolver she kept under her pillow. The text allows one to read the horror as horror at her own gesture or at the life he offered her, to be "one," and the novel does not choose.
What is known is that Moscarda defends her: he tells the judge the wound was accidental and declares that the fault was not hers but his, from the last experiment he let himself be drawn into. Anna Rosa is acquitted.