After Discord spread chaos and Sombra spread fear across the land, the land become hard to live on. It hurt Nightmare Moon heart, seeing ponies hard daily struggle. Nightmare Moon is not evil at all. All she wants is so ponies can live strong and happy. If there's the need to play an evil role, she would gladly do it. for her children of the night, any price she would pay. Therefore the Element of Harmony aren't needed.