I never completly denied that Luna get possessed by dark evil powers. But I also see that Luna more or less willingly choose herself to go this path of revenge. And I still think that she still have these negative feelings with or with out dark powers that got her possessed, especially after all she been through and her 1000 years imprisonment on the moon is something that one can bare or accept.
Of course she regrets of what she became back then, but I'm also sure Luna still holds some grudge and jelousy on Celestia and fact she was banished for millennium losing all her old friends and world she knew. It's not that easy to accept and forget.