And before you ask why, it's because she's a vampire and I'm a vampire hunter. I wanted to play as a good person kill undead and I didn't even join the theives guild and I destroyed the brotherhood. So she's a vampire, she hunts people, drink blood. And I can't kill her?
I kept hoping the dawnguard would give you a quest to kill her eventually like the blades did (SPOILER!!) when they ordered you to kill Parthunaax which is your friend and the reason you were able to defeat Alduin. But no, no quest, no option. I can't believe the blades gave you a quest that you can't turn down and the dawnguard just welcomed Serana.
And how come she's forced in your party the whole time? You should at least be able to leave her and go your quest alone. Instead having to listen to her chatter all the time!
What do you people think?