I Know What You Did Last Summer (Pokémon Edition)

The Kalos Elite Four have really messed up! When they accidentally run over and kill Pokémon Champion Diantha, and all of her Pokémon, they cover up the deaths, and agree to keep silent about it. A year has passed, and now a masked killer is hunting them all down and killing them and their Pokémon. Did Diantha survive and come back for revenge, or was there another witness?