Depends on how you view vampires. Do you see them as evil creatures that need to be wiped out as the Dawnguard believe, or do you see them as an elevated race? Molag Bal created vampires, and the Daedra are the pantheon of gods in the Elder Scroll universe. Serana certainly demonstrated to me that vampires are not always bloodthirsty creatures that seek to kill you. I found the vampires intriguing and joined them my first time playing Dawnguard. I found that being a vampire doesn't mean you have to become a demonic creature that kills everyone, although you can do that if you like even without being a vampire.