January 19, 2016 4:20 PM EST
You bring up an interesting point about how the only vampires we're sent to kill are violent, and this got me thinking. The dawguard has to have spies, they find these violent vampires after all. They also have to be pretty good spies, because they can spot a vampire where others see a normal being, like among a Jarl's court.
So why then do we not kill Sybille? Surely if we show proof she is a vampire we would be forgiven for killing a court Mage, so why aren't we sent to kill her? Because she wants to live in peace. She even has a distaste for her kind somewhat, evidence by the quest she gives you, and there aren't very many tell tale signs of a rampant vampire in solitude.
At the same time though, she does help the leader or red water den get into skyrim, but does little else to help him. But that aside, I do believe that the dawnguard do sort through vampires, killing those that are a threat, and leaving the "Serana's" alone.