I understand the TV show and the book diverge with the book saying she did the double cross for a marriage to a knight and a title. The TV show didn't flush out any of her motivations for her latter actions. There was some foundation that she was in "young love" with Tyrion and not being sensible and refusing to understand or accept the political game that put her in danger. Inexperienced confidence works for her character, until the double cross, which has no motivation justified in the TV show.
If they had spent screen time where she laments she doesn't have a title, or if we see her scheming to get a title or status, it would give her some reason for the double cross. Or... if she had motivated by coin that would explain the double cross, but not the refusal to take the retirement package.
Right now the TV series arc feels unearned, and kinda just fodder to dump on "everyone betrays Tyrion" bandwagon
Maybe I'm missing something? What where her motivations?