Oh! I see where you’re coming from - it’s quite a common notion!
It is a bit of a misconception, though.
Now, I’m no expert, and all the details have been kept very quiet (Which has only made this sort of misunderstanding easier to make, in my opinion! I laid out in my letter to the Palace how it would be better to run an awareness campaign, get the Princess out for public appearances… oh, sorry, I’m rambling :( ).
Anyhow! From what I understand, yes, Princess Luna and Nightmare Moon were the same pony - in a sense. I gather Nightmare Moon was some sort of magical construct that consumed Princess Luna, and was rightfully banished to become the Mare in the Moon. After her return, though, Princess Luna was restored as herself with the help of Princess Celestia’s student and her friends.
So, you’re both correct, and not! Nightmare Moon was a very bad pony, but Princess Luna is, in a sense, not guilty of her crimes (though some argue that point!). As such, I do fully support her claim as Princess of the Night!
Of course, even if Princess Luna was the same as Nightmare Moon, then your objection seems to be more about the efficacy of penal terms and the lack of possibility of rehabilitation, which is a different argument altogether!
And if your objection is to having another Princess automatically ushered into rule - well, I admit I have some difficulties with the theory of Divine Right of Princesses (and, in fact, support the idea of a more republican style of government!) But, for now, at least, our government is benevolent, and works for the benefit of most ponies (albeit with more bureaucracy than I’d like, but I don’t think that would change, no matter what system we had in place). So, as it stands, Princess Luna is one of our rightful rulers under our current laws.
Also, personally, she’s a very nice pony. Very, um… Understanding. She paid attention to me, and clearly did her best to diffuse the situation and make me not feel like a complete foal.
(I… I still felt like I was going to die of shame, though. I will never wear socks again :( )