Chief Pone 
"[@Lord Mega":](/images/2129171#comment_9276348
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She didn't become evil at that moment she was just extremely mad because she didn't believe what pinkie's predictions were true so she raged quit but if you hurt Twilight's friends and you were a villain Twilight will basically fill up with rage and disintegrate you.So basically that was the show just being funny since Twilight doesn't have the power to control the sun like Celestia which is the princess of the sun and can turn into Daybreaker when boiling with rage but that was never shown having Celestia get furious and transform.That was just my guess since Luna turned into Nightmare Moon because she was sad and furious with ponies not caring about her and only showing affection to her sister.
Master Chief Pone
She didn't become evil at that moment she was just extremely mad because she didn't believe what pinkie's predictions were true so she raged quit but if you hurt Twilight's friends and you were a villain Twilight will basically fill up with rage and disintegrate you.So basically that was the show just being funny since Twilight doesn't have the power to control the sun like Celestia which is the princess of the sun and can turn into Daybreaker when boiling with rage but that was never shown having Celestia get furious and transform.That was just my guess since Luna turned into Nightmare Moon because she was sad and furious with ponies not caring about her and only showing affection to her sister.