Spoilers again!
Silent Dusk/Twilight didn’t want to do anything bad.
Silent Dusk actually only had one thought.
Show your teacher [Celestia] that you will achieve your destiny [of succeeding her].
Raise the sun.
And share it with everyone.
Whether or not plunging the sun into the planet is an evil or villainous thing is up to you. She just became her destiny.
Or at least what Celestia wanted Twilight’s destiny to be.
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Oh man, these are fun! You guys should keep asking me questions! :DD

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