Can't think of 10 off the top of my head but the ones that do come to mind right away:
Nurse Ratched from One Flew Over the Cuckoo's Nest
Scar from The Lion King
Amon Goeth from Schindler's List
Cruella de Vil from 101 Dalmations
Percy Wetmore from The Green Mile
The Joker from The Dark Knight
Ignoring a lot of the super obvious slasher movie villains because while they're terrible, I don't find them all that realistic. I find the more subtle dark characters to be more disturbing because I can see real levels of humanity in them (except for maybe Scar, haha - he's just too horrible to leave out!).