Why Did Eric Really Kill Adrianne?

Eric Copple was found guilty of killing his fiancé Adrianne and her roommate Leslie on November 1st, 2006. He claimed the murder wasn't in the least bit personal, instead blaming his bloodlust on the death of his grandfather. Lily confirmed that Eric's depression was to blame for his behavior, but did she know he had killed them both as some kind of punishment? It is impossible to tell until Lily-Coppel confirms such a theory; it remains pure conjecture in the eyes of the law.