In the Rainbow Factory...

"Get her out. And Hide, remove the cloths. They were never necessary in the first place."
Two pegasus ponies flew over to Rarity and Applejack's cage and one of them unlocked it. Instantly, two of them zoomed in to hold Applejack down. One of them went in to grab Rarity. She looked for a way to escape but there was none. She attempted to defend herself but failed too when the pegasus dodged her attack and grabbed her from behind. She was dragged to the center of the room and Rainbow Dash landed next to her. The other two pegasi in the cage quickly zoomed out and one of them locked the cage.

"So... I think I have an idea for you. But first- tie her hooves up!"
One of the pegasi grabbed a rope from seemingly out of nowhere. The other pegasus held Rarity's hooves together and together, they tied her hooves up. Rainbow gave a disgusted look and shooed the workers away as soon as the finished. Rainbow Dash grabbed the kids she had used for Pinkie Pie and put it back onto the cart. She grabbed another one, sharper, longer, and slightly bigger, and walked back over. Rainbow Dash threw the knife and hit Rarity in the stomach. Rarity struggled to break the ropes. She used as much force as she could and managed to lift a knife off of the cart. She threw it towards Rainbow Dash. It hit Rainbow on the back. Unaffected, she withdrew the knife and held it against Rarity's neck.

"Good try."
She inserted the tip into Rarity's neck and flicked it up, leaving behind a trail of blood. She put a hoof to the slash and applied pressure. She threw the knife aside and smiled.

"Do it again. You might be stabbed in the back five times, just like what you and the others did to me."
Rarity struggled to break the ropes. She turned onto her side and Rainbow let go.

"Aw. Trying to escape, aren't you? Just like Scootaloo did. But failed to."
"Wait. You killed Scootaloo?"
Rainbow Dash stayed silent. Nopony moved. Suddenly, Rainbow Dash grabbed a knife and stabbed Rarity on the head. She pushed down on it and withdrew it. She dropped the knife. Rarity tried to stay conscious. But with the hole in her head, it made it difficult.

"Death... is all you horrible ponies deserve at this point. There will be no regrets."
She picked the knife back up and with a quick slash, the unicorn's head came off. Twilight screamed in terror and covered the sight. Applejack just sat there in horror.

"RAINBOW DASH!!! WHY DID YOU DO THAT?!?!"
She didn't answer.

"Get Applejack out. I think I know what to do. Twilight. You're next."