They gave Laurel's relationship with Ollie two strikes from the get go with the "cheating with her sister" theme.That makes it all the more compelling that with Sara back (and her history with both Sara and Ollie being a factor) that Laurel would turn villain.
You know, I could get on board with that. KC would probably make a great Lady Laurel Slade. He seems like the kind of man that would make her feel appreciated.