(Fem or Gender Neutral)
And because he's enjoying Obi-wan's fear at the moment. Vader this whole episode has been good, he's been a Horror Movie villain, slowly stalking after his prey. That's damn good Vader.And I highly suspect the reason why he just stands there and lets him go is because he's so disappointed in the moment and in Obi-Wan.
The scene on the truck where Leia realizes that Ben knows her real mother and asks him if he's her dad breaks my heart every time I see it.