in "sweetheart of song Tra bong" what transforms mary anne into a predatory killer?

Check this GradeSaver link out,

http://www.gradesaver.com/the-things-they-carried/study-guide/section4/

I think what transformed her into a killer is her curiosity. She explored and wanted to learn more and more. She eventually thought that the Vietnamese way of living was more suitable to her nature and decided to leave Mark Fossie and became one with the land.