After Rainbow Dash's 'death':

•Twilight left Ponyville and returned to Canterlot the next day, in order to 'keep herself safe'.

•The police continued their investigation on the murders of Fluttershy, Rarity, Pinkie Pie, Applejack and Rainbow Dash, but Twilight was fairly sure she got away with it.

•Eight months after she had returned to Canterlot, all of their evidence fit together, which concluded the investigation. In the end, they proved Twilight guilty of murder.

•Oddly enough, Twilight confessed to it all.

•The normal punishment for such a crime would have a pony sentenced to death, but Princess Celestia didn't have the heart to have Twilight executed, so instead, she had to think of another punishment...

Author's Note

As for Princess Celestia's "other punishment", use your imagtination. Leave it at that.