Anon with his bigp
 
Anon: “I won’t let you harm her!”  
Celestia: “We have no choice Anon, she is a danger to all of Equestria!”  
Anon: “no u”
 
I had a discussion with a friend about what would happen if Twilight turned beast. What would Anon do? What would Celestia do…etc.  
If her mind were intact she would surely want to die, but she was suicidal before that. Anon would not be able to let her go, just like she would not be able to let him go. Even after she has turned into a beast there is enough of her love left to feel her connection to Anon. Anon would have lost his mind at this point as well, blaming himself for her condition and the possible doom of the world. Celestia and Luna now face the most dangerous duo they could imagine. What magic could defeat their love…?
 
If you are wondering why they have to be killed. Twilight’s corrupt magic might spread through equestria and cause world ending scenarious. but that’s lore for another day.