Sorin interrupts Avacyn's attack. She recognises her creator and confronts the contradiction in her own actions: a guardian has killed innocents. She condemns him rather than accepting his authority as proof of goodness.
Their fight drives them through the cathedral floor into the cellar where she was made. Avacyn finds a wing-shaped mark left by the creation spell. Sorin offers to cleanse her mind and make her a virtuous instrument again, but she refuses to submit to his control. He cannot simply replace her or build another Helvault.
He reverses her creation and unmakes her. The immediate attacker is gone, but so is Innistrad's protection. His grief and insistence that it was necessary cannot erase the consequences.