From a writers standpoint how would you have handled the Mary
Marvel situation in the 52 series from 2007? Why can't an inherently
good character become permanently evil? -- Patrick Scott
I'll take both parts of that separately.
I would go back an redo the story so that Mary didn't turn evil in the first place. It just didn't fit the character. To me it was one of those "let ramrod a character into a story" type of situations, rather than a "let's plot in a way that begins from the character" situation.
As for good characters becoming permanently evil, well, in comics, nothing is permanent. I used to think as least some things were, but short of Uncle Ben being dead, nothing seems sacred any longer. I have to admit that I much prefer the reverse of that story -- the evil character becoming a changed person. Perhaps my favorites in this camp have been Mach V and Songbird of the Thunderbolts, Sandman from Spidey, and Heatwave from the Flash's Rogues Gallery. But even some of these have been reverted back to the criminal ways now.
Having said that, I have no problems with good characters becoming bad guys (or girls), as long as the story warrants it. Case in point, Hal Jordan. After having his city destroyed, if anyone was in a position to let vengeance and anger and insanity take over his soul, it was Hal. I loved Hal as a villain. He had so much more depth that way.
But the trick with comics marketing is maintaining the status quo, so after a few years, he had to be back as the main Green Lantern.
I guess that's the answer to your question then, Patrick. Why don't they stay evil (or stay good for that reason)? Because of the compelling and overriding power of the status quo and how it must be adhered to in order to market characters to the masses.