From the artist’s page:
 
“Dear Princess Celestia,
 
Today I learned that there is only one kind of magic, power. Nopony can gain magic ability and strength through making friends, one must take it upon herself to study and reach her goals in this life. My studies on ‘friendship’ have only served to prove to me that friendship isn’t magic at all, it is corrupt and filled with hardships. So why bother continuing with such trivial studies, when I can just simply cut out the middle mare?. I became your student in order to learn magic, not so that I could run around cleaning up all of your past messes, which YOU were to incompetent to finish yourself!. HAH! and don’t get me started on having to listen to and follow each and every one of your stupid little demands!.  
Well, I suppose the past doesn’t matter any more does it?, Since I am now the most powerful unicorn in all of Equestria and I have surpassed even you, my teacher. With this last spell, it shall all be over for you, my ‘former’ princess, and Equestria shall answer to me!. But don’t worry, teacher some of the wisdom that you passed down to me shall live on, Oh yes, those ponies shall adore me, be happy and become my ‘friends’…even if I have to MAKE them! hah hah. Equestria has a new ruler now and unlike you my teacher…in my empire, there, will, be, NO second chances! hahahahaha.
 
Good Bye Teacher!! * KABOOM*…………………………………………………. yours sincerely Twilight Sparkle”

Man, i so wanna see an evil Twi episode.