But of the most blessed of all, when this now Un-Dead be made to rest as true dead, then the soul of the poor lady whom we love shall again be free. Instead of working wickedness by night and growing more debased in the assimilating of it by day, she shall take her place with the other Angels.

– Bram Stoker

Dracula, Chapter 16. Van Helsing to Arthur Holmwood, on freeing Lucy Westenra from her undead vampire state and re-uniting her soul with God. The purpose of killing Lucy is to restore her purity and make her one with God.