If you don't, you can't get her to cure herself. You have to talk to her a certain way during the quests. You can get Serana cured, but it's a pain in the butt for almost no reward (She keeps many of her vampiric powers like the drain spell and vampire seduction). Can you cure serana If you are a vampire? Vampires are not superhuman beings like you seem to think they are.
She would put up no more of a fight than they did. The player kills a dozen as he crawls through the cave to find her. Serana is no stronger than any other vampire.
