The evil that turned Luna to Nightmare Moon was not just from within. Despite anypony's good heart, it could affect them and turn them. The Corruption was spreading fast over Equestria, and it reached the most powerful victim ever. Princess Celestia.
It was not just Luna who had a bad side. The difference was that Luna, even as Nightmare Moon, could be defeated. Celestia could not. there was nopony who could challenge her in this state, and nopony could keep the world from burning.