I've read a fic in which Drusilla turns Giles into a Vampire during Becomming Pt 2. It's not exactly Giles as a big bad though, or at least it wasn't when i was reading it. Incomplete and hasn't been updated in a while, if memory serves the author was focusing on uni.
no subject
http://www.fanfiction.net/s/5738904/1/Still-Here