I read a tie-in novelette once, where Giles *had* been turned into a vampire.

He kept his academic-trained mind. He arranged for Buffy to be captured and held in an in-escapable stone cell, with food and water etc. provided, without trying to harm her, on purpose because killing her would have just activated a new Slayer and this way he had the only Slayer in the world safely neutralized.

And outside that cell, the vampires ruled all of Sunnydale.

It was deeply scary to see what an organized evil mind could do.