Mary Marvel is such the quintessential good girl that certain writers and artists over the years have been tempted to concoct a heel turn for the character. Although such impulses usually result in less than memorable stories (to be charitable), the concept of a “Good Supergirl Gone Bad” did make for a few interesting twists and turns during Mary’s Golden Age heyday.
(It probably helped that the Mary Marvel of the 1940s was one of Fawcett’s top characters, rather than an “underused property” needing some sort of gimmick to gain the attention of an increasingly jaded modern comic audience.)
The twist in this particular story is that Mary Batson is corrupted and subsequently wages war against her virtuous alter ego, Mary Marvel. And yes, in this particular story the two Marys are depicted as separate individuals. It’s even spelled out on the original comic book cover …
“The Milk Of Inhuman Kindness” originally ran in Wow Comics #17 (Fawcett, Sept. 1943). No writer’s credit this time, but the artistic honors go to long-time Mary Marvel contributor Jack Binder.