Okay it's very simple, its the butler. Every murder mystery book, movie or tv show the butler is always the one who is the killer. It's always how these end, now they've done a good job swerving the audience this whole season. It will be the butler though, always has been and always will be.
I just think the way they ended it with Giles being in a trap and having to be saved reminds me a lot of SAW, when the person they go to save is really the killer. Add that in with his "ankle monitor" to get the audience and characters to believe he's trapped with them and my butler theory. It just adds up and they could do a second season, just at a new place with a different killer. heyitsmaggee