Well, it's clear to some degree she wasn't herself. Perhaps its simply a case of
With Grea T Power Comes Great Insanity? With all that power she thought she could just push and push, take what she wanted for just a little while. Then when Celestia had to banish her, she felt wronged, because she only wanted a little respect and she was banished for 1000 years. So by the time she came back she really
was evil, and Twi and the Mane 6 just knocked her down to size, and gave her a little bit of a timeout to think about it. Or maybe she took it from a nasty source that kept whispering in her ear, egging her on to make some really bad decisions and clouded her mind so she couldn't really understand what she was doing until she was free.
Personally, I'm trying to forgo theories; I just want to see a flashback episode of the whole thing. It's highly unlikely to happen unless something
did corrupt Luna, in which case it's likely to be a season finale thing, but still. It would be nice to not have the different sections of the fanbase sniping at each other over their various fanons, completely regardless of the fact that they all have the
exact smae amount of merit, since they're all fanon.