Exactly, which is why I said I understood why you reached that conclusion. However, she specifically said she gave him quantities of poison designed to make him think the arid environment was making him ill and drive him to fairer weather. Upon his return from the coast he was unhappy with what he observed. Specifically the debauchery and criticized her for not bearing a son. Her friend was brutally murdered and her father in law accepted gifts and dealings with the murderer. He demanded his son dissolve their marriage. Under the imminent threat of the destruction of her lifestyle she poisoned the man. Also, note how disgusted she is with her first sexual encounter with Crixis which was specifically motivated by a desire to get pregnant to produce a son.
She started using drugs, engaging in immoral activity, and committing murder all during this season. The initial indications in the first episode was that she was not inclined to that sort of behavior, or had shunned it, until her friend arrived. By the time we get to Blood and Sand she is quite the evil wench but we she her decent in the current season.
"I would rather claim to be an uneducated man than be mal-educated and claim to be otherwise." - Wookie 03:16