Found another really good fanfic to give you the feels a few months ago. It's called The Keepers of Discord. it takes place after Return of Harmony and acts as if Keep Calm and Flutter On never happened. it spans out across many years and is all from Discord's point of view while he is imprisoned in stone. there is also a reading of it with artwork on DRWolf001's channel
So i don't fully believe in this theory, but was mostly amazed that i could come up with something like that.
I think that if this did happen though; Luna always had enough power to change if she really wanted to (or at least knew how to get the power), and even thought about it more than once, but didn't because she loved her sister and knew she had a duty to protect Equestria. Then the sirens came along and sung their song, which starts to infect Luna, clouding her judgement and making her ever more resentful. She was able to hide this for a while after Starswirl banished the sirens, until she felt the moment was right.
Oh, and Icy Shield, i do agree that it was mostly her fault. Outside forces or not, she was hurt and angry, which lead ponies to not always think about the consequences of their actions.