Rules:
Stay away from fanon. Remember, the canon cannon always beats the fanon cannon.
*This rule can be omitted if it's for characters without an established personality, such as Luna. Note how I said Luna, not Nightmare Moon
No copy/paste, except for quotes and sources.
My position:
Celestia is a kind, benevolent ruler. What she did to Nightmare Moon was necessary for survival.
1. Nightmare Moon openly refused to take down the Moon, and can thus be deemed a traitor to her country.
2. Let's Say Celestia hadn't banned Nightmare Moon inside the Moon, plants need sunlight, don't they? Ponies are Herbivores, aren't they? So if Celestia hadn't found a way to take down the Moon, and bring back the Sun. All plants would die, and start a famine within Equestria.
3. Celestia has shown time and time again that she is willing to sit down with her subject. Such as in episode 22 when she does have a "casual" get-together with everypony, only too be interrupted by Royal duty.