Maybe she’s friends with Mercy already, and she pretends that she doesn’t know about Jared, and when Mercy tearfully confesses that they’ve been seeing each other (omitting the detail about the cheating/overlap), Sarah acts like she’s totally forgiving and just appreciative that Mercy told her, and tells her that she really just loved Jared as a friend and is happy for them. In the meanwhile, she starts gaslighting Mercy in a subtly escalating game of psychological warfare. Ultimately, Mercy is driven to extreme anxiety and paranoia, and strangles Jared in a fugue state. The story ends with her being carted off to a mental ward in a prison.