So....when you join the Dawnguard. After you beat the main quest and kill Harkon. How is Serana so valuable that Isran and the others allow her to live?

Isran places her in your care and states she's an asset. Well...after the main quest....isn't her use....done? So wouldn't that make her now expendable?

Even before you kill Harkon he even brings this issue up to you. I honestly, as a vampire hunter would kill her off rather than bicker with her or give her an ultimatum to have her cure her own vampirism.

I think Bethesda should have given us that option. The only time that the Dawnguard does want her dead is when you side with the vampires. But......not if you join the Dawnguard and after the main quest....after they raid the vampire's castle.

thoughts?

Because Isran learned to temper his hate-on of vampires with discretion. Note how he starts treating her like a person instead of a thing, offering condolences to her when she has to take part in killing her father.