Betrayals are by definition horrifying and fascinating, even in fiction, because you cannot have betrayal without first having trust. Without first developing a meaningful relationship between the characters you intend to have betray each other, your clever plot twist is just common cruelty.
She lists a few motivations for treason (eg. fear/temptation), and lists appropriate examples from literature, and they're worth looking through, even to find new reading material for your own work.
At the end Susan lists a few flash fiction exercises that might help your writing too.