Wanna know why? His plan literally breaks Sora and even makes him almost become a person he isn't. His plan destroys Sora's mind, makes everyone including his friends forget him. and one point in the game he actually had Sora beat. Even after he gets defeated, he still in a way won. The effect his plan had still goes on in 358/2 Days and in KH2. Sora wakes up and is still barely gaining back everything he remembered. So if you look at it, Marluxia was Sora's greatest enemy.