Plot: Sigma has sent Double the spy after the Mane 6, to spy on them and keep them in order from beating him. When the Mane 6 got the Elements of Harmony artifacts, Sigma knew that he had to do something and quick, so he ordered Double to kill the Mane 6. Rainbow Dash was sleeping at night time, when she heard screams which sounded exactly like her friends. She rushed to where the screams were coming from, Twilight's Castle, and she couldn't believe what she saw, Double, their once called friend, was standing in front of their corpses, in a different form then his usual appearance. That was when he noticed Rainbow Dash and turned around, a big smirk on his face. Rainbow Dash was devastated, crying, and filled with a unspeakable amount of rage and fury. Double said if she wants to live, to meet him at the Final Weapon, a giant space station in orbit which has a giant weapon built into it which can destroy planets. Rainbow Dash mourned over the death of her friend, before flying to the Final Weapon.
She would do whatever it takes, to watch Double die in the maximum amount of agony he can experience, to avenge her friends one way or another. She is Loyalty, but also, Justice.