This is my list off the top of my head of every time a Magical Girl has been turned evil, due to a thing that came up in a chat I was in.
Warning, there will be spoilers. Also this doesn't count if they started off evil, just if they were good and then become evil.
All the Sailor Senshi in Sailor Moon except Moon and Chibi-Moon:
were turned into the Galactica Senshi.
In PGSM, Sailor Mercury:
was turned into Dark Mercury:
Sara the Orange Pearl Princess:
Warrior Pudding (NSFW series):
turned into Dark Pudding:
became the Devil:
was briefly mind-controlled:
I think that's it for series I know.