From what I remember from the previous issue, Mary said that she tried not to change back, but Desaad kept yelling her her from inside her head until she caved, or something.
That or she missed the power.
Making this the... third time post-Infinite Crisis that she's reformed from evil, only to cave again a few issues later.
On the bright side this particular sordid littel storyarc is meant to be finishing in this issue, hopefully meaning that Mary's possible reform'll stick this time.