maddy mary ann spoiler question

Ned speculates on that in one of the last scenes in the movie, at the prison with Oscar. He says that Matty/Mary Ann (Kathleen Turner's character) might have had something so bad in her past that if Walker found out he wouldn't marry her.

That's as close as we get to an explanation.

And you wanna know the worst part? You're from outta state!

She was worried Edmund would find out about her'past' and not marry her

I always thought she needed another identity to assume, after she left the country with her money. Since "Maddy" was killed (the real Maddy, who everone thinks was Turner), Mary Ann can leave the country and live with her real identity. Can't have a paper trail in the name of someone who is supposedly deceased.

She also needed another body that could be used as hers, to frame Hurt for killing her. She couldn't frame him for just murdering the husband, since he would've told them she was involved, and she would have been arrested.

This way, she, the wife of the rich murdered man, is dead. No one will be looking for her (as "Maddy"). So she lives a life on foreign grounds, using her own legit IDs.

Perhaps they weren't 'friends' as much as business partners. Maddy came to mary ann and might have said I've got some rich older guy (crenna) dangling and if you do this for me, I'll pay you X amount of dollars