There is a element most don't think about ever. I don't actually think Nightmare Moon was truly evil, don't get me wrong what she was going to do would have been terrible. But I don't think Luna or Nightmare Moon was fully evil.
Notice when she first appeared she defeated the guards without a second thought, and barely used her magic.
Then she set simple ways of dealing with the ponies, which were at best more of a test than anything else.
If anything I think she wanted to be beaten, or at least have some fun. Not fully sure though.
Though in a slight after thought, what would have happened if Twilight had tried to explain and help Nightmare Moon rather than use the elements on her.