I would have thought that after a thousand years of brooding Luna would not be so quick to forget the banishment.
I think that Celestia acted to save her subjects, but Luna clearly acted on her jealousy and rage.
After a thousand years of isolation with nothing but those thoughts it's no wonder Nightmare Moon was so unreasonable. That sort of resentment does not go away easily.
It's a hard decision, to banish your kin. No matter how necessary and how many lives it saved it must have torn her up inside.
Addendum: I've never understood why people think that Luna would champion a Republic, since that implies at least a level of democracy. Luna seems like she'd strictly follow tradition, and a Republic seems very different from the Monarchy that I assume Equestria was run under for such a long time (and still seems to be). In fact, Nightmare Moon dubbed herself as a Queen, and even if you don't think Nightmare Moon was the same entity as Luna, then at the very least you must admit it fed off her desires. So the notion of a "Lunar Republic" does not make sense to me. If any of the princesses were likely to champion a democratic form of government I would wager it would be the younger ones - Twilight and Cadence rather than Celestia and Luna.
"Draw a pony broken up/Draw a pony mourning"Celestia is best princess