anouk's crime (spoiler)

I think neither. Michelle tells Noemi that Anouk killed her lover *after* he left her and returned to France, and that Anouk killed him in a jealous rage. I think he'd found someone else, and Anouk couldn't stand it. After all, look at how passionately she loved him--she abandoned her husband and her own children (a hard thing for any mother to do). She didn't want to lose him, but she probably had the 'if I can't have you, then no one can' mentality. I think she lied to Michelle when she claimed it was self-defense, just as she lied about being Jewish and hid the paintings. Later, at the Cave of Shem and Ever, she admits that she tried to blame others for Joseph's death, but she realized it was her fault.
But who knows? Perhaps in Anouk's odd sense of reasoning, killing him was the only way for her to survive--to survive the heartbreak of being abandoned, to atone for the 'sin' of abandoning her family and her beliefs. So then she could rationalize that it truly was 'self-defense'.

and while BDSM can employ a plethora of devices and such, it rarely involves anything that would cause the blunt force trauma that Michelle reads about in the newspaper report. If he had choked to death, I would agree with the bondage-gone-bad angle, but to go from role-playing to bashing someone's brains in is too big of a jump in logic for me.