This adaptation is based off of a scene in Charlie's Angels: Full Throttle where antagonist Madison Lee confronts Charlie, via speaker box, about what she has done and why. I felt Evil Sunset Shimmer and Princess Celestia seemed to fit this moment so here we go.
(CELESTIA walks through the hallway and enters her study. A fire is lit and a pony is lying beside it)
Celestia: Twilight? Is that you?
Sunset: Good evening Princess. I love the new décor.
Celestia: Sunset Shimmer! Where's Twilight?
Sunset: I'm sure there's more where she came from.
Celestia: Sunset, what have you done?
Sunset: Oh, don't lose yourself over this. I'm just having a little fun. Besides, even if I did do something to that replacement, you never had trouble finding somepony willing to give their life for you.
Celestia: Hurting my students does not prove your the best. You know that.
Sunset: Do I?
Celestia: Sunset, you never fully understood what it meant to a part of my school. It's more than about hard work. It's about teamwork, making friends here and--
Sunset: Yeah. Yeah. I know the sales pitch: One happy family. It was very seductive. Once.
Celestia: Sunset, when I made you my personal student, I wanted to show you what you're truly capable of, but instead you used your status and your magic to fuel an ambition and put others down just as they had done to you.
Sunset: Why not? It proved they were wrong about me. But if that wasn't enough, you kept me away from the real magic because you're afraid of someone taking your place. So much for friendship.
Celestia: You're not as powerful on your own as you think. Do you remember when you deliberately went rogue on a group assignment involving a dragon and her eggs which put those other ponies in danger? You didn't listen to my instructions to use your individual talents to send her away. Instead, you used your magic to force her away which made the dragon angry and everyone got hurt, including you. When I finally got you out of there, you were wounded so badly I thought I going to lose you. That's should've shown how much I care about you.
Sunset: You did lose me. I learned something that day. Being your little protégé wasn't fulfilling my destiny. It was keeping me from it. Much how you kept that portal away from me. I learnt much from that other world. That I am a powerful leader, and I'm meant for more than you offered me.
(SUNSET prepares to leave.)
Celestia: Your destiny is yet to be written. Do not do this Sunset. Taking the lives of innocent creatures will not make the pain go away. I know you're better than this. There must be something you that's still good.
Sunset: I was never...good. (Turns to CELESTIA. Tears begin to run down SUNSET'S face) I was great!
(CELESTIA gasps. She finds herself awake and in a carriage with LUNA on the way to the Crystal Empire to the Princess Summit.)
Luna: Sister, are all right?
Celestia: Yes. I just had a bad dream.
Luna: A bad dream or vision?
Celestia: I hope it was only a dream.