In another world...
I'll give you a fair warning that this may be bit of a tear-jerker, but here's another pony picture, also featuring Night Mare Moon - Ende26's In another world...:
Image: Ende26 @ DA
What if Celestia had really killed Night Mare Moon, way back when? Certainly Night Mare Moon was trying to kill her, and though I believe that Celestia, knowing that it was still her beloved sister in there somewhere, would try not to hurt Night Mare Moon, well - a battle is a battle; even self-defense may be deadly, and Celestia had to find a way to subdue Night Mare Moon and banish her. Mistakes are easily made, and some may be permanent.
What if, indeed? In another world, Night Mare Moon died, and Luna with her. Given how grief-stricken Celestia already was to have had to banish her sister, I'm sure she's ever-grateful that it isn't hers.