Read "Can-Can" by Arturo Vivante
You'll find this story in the preview for the anthology Sudden Fiction, edited by James Thomas and Robert Shapard. This is the second story in the anthology, right after Grace Paley's "Mother."
I highly recommend the book, but for now, you can read Vivante's story in the Amazon preview.
The prompt: Write a story about a betrayal from the point of view of the betrayer. Include a twist at the end.
Your betrayal can involve a love triangle, as in Vivante's story, or it can be a betrayal of some other nature--in business, family, or something else.