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Or is there? Anyone? Killing him seems so useless.

I presume he'll fight you if you don't tell him he's offspring of a vampire (or simply don't accept that quest). It is only when you inform him of his heritage that he becomes depressed and loses the will to fight.


I am in blood
Stepp'd in so far, that, should I wade no more,
Returning were as tedious as go o'er.