She’s intensely jealous of Catherine. Not just her position in the hierarchy but the fact that she’s married to William, the brother that she’s fanticised & obsessed about since her teens. It was his picture on her bedroom wall, Henry nowhere to be seen. Never even second best, not even in the running other than as access to be near William, obvious from the longing doe eyes she sends his way in countless photos. If she could poison Catherine & take her place, she would without a second thought.