nmcil:
wish I could remember the title for this one story - it's not one where Spike kills her but where he is attempting to send her back to heaven - she asked him to send her back if he could. It's a excellent story - angst because she is so miserable and Spike will lose her again but it's what she wants.
http://bloodshedverse.com/stories.php?go=chapters&no=12645The Ghost in You by BuffyMeetsSpike
Goes AU during Bargaining. Willow's spell goes awry, leaving Buffy incorporeal, and visible only to Spike. With the Scoobies convinced that Spike has lost it, how can he help Buffy get back where she belongs?