Sunset, she was implied to have her vicious personality, as a result of being deprived of love and friendship, as she said she has no idea about friendship at the end of the first movie. The Holiday comic from the previous year, also gives more arguments for this, as Sunset's says her family NEVER celebrated Heart Warming Eve with her. Which implies that despite their daughter's talents, her parents were heavily negleceful at the very least. Another interpretation, is that Sunset considered Celestia her mother figure, but Celestia didn't recognize it, and didn't act because of that.

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(to be clear, I'm not a Celestia basher, but seeing Celestia's long life, she might have started to become more distant, due people around her dying, and her being a mother figure for all ponies).



It's an interesting theory I think,

as bullies quite often(but of course, not allways) come from negleced or abused backgrounds.

Also, returning to