Does anyone know any fics where Buffy turns evil, excluding cases where she turns into a vampire? Thanks

Roundabout - Devils Piglet
Summary: Buffy channels her inner psycho, sending Dawn and Spike on the run. But homicidal mania never stood between Buffy and Spike before. Rated: NC-17

You can find it at Sinister Attraction.

There is definitely some darker Buffy to varying degrees, in the following ones, (I'm sure canon Buffy would count it as 'evil' ). I think all relate to a slayer demon side...

Blood Bound by DreamsofSpike
Early Season Four, immediately after Spike is chipped, Giles finds an ancient ritual that could solve their problem of what to do with the not-so-harmless vampire...but Buffy ends up getting a bit more than she bargained for in the process.

Buffy is also stepping more towards a darker nature in...
Normal by Kari Mouke

and very much so in this...
A Not So Gorgeous Mistake by Kari Mouke

Her darkness in this gets broken down eventually, interesting fic bit different...
Black Widow by bogwitch
Dark futurefic. Buffy has been missing for several years. When Spike goes to find her, he's in for a very deep shock.