Devious Maids Season 2 Episode 7 featured some shocking moments that revealed more about the Deerings and their secrets.
Marisol was still convinced that Dahlia and Opal carried on an affair. She basically demanded that Nick fire Opal and couldn't understand how he would want to live with his dead wife's lover still there. It seemed like a fair request.
Rightfully so, Marisol was upset and gave Nick an ultimatum: her or Opal. Through a flashback to the night Dahlia died, we found out that Opal was the one who pushed Dahlia. Flashback to the present day and Opal ran Nick over.