The only reason whe is allowed is because she is the only lead they have. Not all vamps eat people. The count of skingrad for example is a great person and leader. Also vamps only need to drink blood and they are good to go. Don't gotta drink a person to death just a little of the top. To me that's not evil just trying to live. Each side has clear evil characters and obvious good ones. That's where my dilemma is neither side feels like the good guys. Even some of the side quests I question.
i never said all vamps are bad but harkon and his followers are.
oh and serana still lives at fort dawnguard after the quest is complete and she's no longer needed...she's not evil and neither is isran and the dawnguard, they're trying to save the world from the evil vamps like harkon.
like i said its pretty black and white