I know Sunset was the Princess’s student but there something between those two that I get this feeling they related like they are sisters or mother and daughter or etc. let me explain by talking about Sunset’s name. Sunset is of cause the sun is setting so the end of the day and Shimmer is faint light. Sunset relates to the sun and that is Princess Celestia, Shimmer is dim light like the stars and moon at night like Princess Luna.
Consider the families have the common meaning in each family’s name like:
Apple family: AppleJack, Apple Boom and Big McIntosh (have to Google the wold McIntosh to make sure it’s a apple. lol) they all got apples in their names!
Next is Sunset cutie mark.
(Ref from http://chainchomp2.deviantart.com/art/Sunset-Shimmer-Cutie-Mark-390443638)
And Princess Celestia’s mark:
(Ref from http://blackgryph0n.deviantart.com/art/Princess-Celestia-Cutie-Mark-209185555) Do you notice that the sun of the two cutie marks look the same but only differences are the colours.
Let’s look at Rainbow Dash’s parents, her mother:
(Ref from http://bluemeganium.deviantart.com/art/Nice-Rainbowshine-368217071)
And her father: (Ref from https://derpibooru.org/242101?scope=scpe46e48ff41f234bc8bb9f0a27692c88b937fed795)
The mother got a rainbow mark and the father got a cloud and a ray of light rain from it (I think it’s a ray of light…).
Look at Dash’s mark:
(Ref from http://mlp-vectorclub.deviantart.com/art/Rainbow-Dash-Cutie-Mark-314408910)
As you can see Dash got the cloud from her father and the bolt of rainbow (just the rainbow) from her mother.
I know Sunset was all bad guy in the first film of Equestria Girls but Princess Luna was once Nightmare Moon. It took Twilight awhile how important friendship is and same goes for Sunset.
In the comics when Sunset saw the mirror gateway to the human world she saw herself as a princess and something else but couldn’t get the chance because Princess Celestia led her away. So this force Sunset to find out what the mirror was showing and broke the rules and research dark magic. Sunset got fund out and got banned from the palace but Sunset runaway into the mirror to the human world.
I don’t get why Celestia was being harsh with Sunset and throwing her out like she isn’t worthy to her anymore. Sunset felt rejected and turn to dark magic.