Anastasia + Christian Grey ("Fifty Shades of Grey")
There is S&M, and there's abuse. It's difficult to categorize the "Fifty Shades" relationship, but since Anastasia admits that she's afraid of the guy, and ends up leaving him eventually, and he seems like a creepy, gross stalker for most of the books, we're leaning towards the latter for this. When someone is young and immature enough to say "Geez" constantly and they've never even kissed a boy before, they're probably not ready to make the decision of entering into a dom/sub relationship.
Jaime + Cersei ("Song of Ice and Fire" series)
Well, they ARE brother and sister, so there's that. But also, they're both super evil. They push a child out a window so he doesn't confess that he saw them having sex. That's pretty messed up (we realize that Jaime is a pretty dynamic character and changes a lot to be not-so-evil, but he sure is a "bad boy" at the beginning).
Charles Ryder + Sebastian Flyte ("Brideshead Revisited")
Some people don't think Ryder and Flyte were romantically involved (we think those people are stupid). Flyte was a crazy alcoholic, Ryder was an obsessive social climber. Flyte also never seems to reconcile his strict Catholicism with his sexuality. He is extremely depressed and reclusive. And then throw in the fact that it wasn't socially acceptable to be gay back then: a relationship doomed before it even began.
Anna + Karenin ("Anna Karenina")
When a book begins with “Happy families are all alike; every unhappy family is unhappy in its own way," you know you're going to have a lot of dysfunction going on.The book is about Anna's loud, public affair with Vronsky. She tells her husband openly that she doesn't love him close to the beginning of the book. She is pretty much a train wreck (pardon our metaphor), and things do not end well for the couple (or for Anna).
Oedipus + Jocasta ("Oedipus")
Oedipus kills his father and marries his mother. Upon learning that they have done this, Jocasta kills herself and Oedipus gouges his eyes out. Yikes.
The rest at the source