A/N: I don't really like Sally Donovan, but wouldn't it be interesting if there was more to her dislike of Sherlock than meets the eye?
She's read about sociopathy before. Not for work, though that was always her excuse when he would ask about the books and articles. He didn't know she was checking off boxes which described him, her boyfriend, perfectly. She'd never seen him violent, but she saw him calculating, and manipulative, and saw that he could lie with such ease. She was desperately in love with him; he would rope her in with apologies and endearments, but again and again she'd find it was to suit his needs. He put himself first, always. Never seemed to care, but could pretend with the best of them.
She managed, in the end, to break free.
Sally never would have taken to Sherlock, that much is true. His casual observations were often rude, and he looked down on everyone with conceit and contempt. But that wasn't it. Suddenly there was someone new that fit so many boxes, and he must be even worse because he's drawn to the morbid murders. Not a sociopath, then. A psychopath. One day, investigating wouldn't be enough.
That's why she warns John. He seems as nice a guy as any, and she knows Sherlock will draw him into his web. She rather hopes John won't fall prey to the same kind of person she had worked so hard to leave behind.