Not long into the Big Bad's gloating about how successful they were, everypony is rightfully confused as to why Pinkie starts laughing in his face. Turns out Celestia has arrived – and isn't exactly pleased to see Twilight's friends being attacked.
Mis-blamed: The entire royal force is often completely blamed for the near-successful regicide of Princess Luna, but this is not the case. Only three of those guards took part of the plot, and all the other guards, specifically the Beam siblings, Radiant Dawn, and Shining Sword, were unaware of the conspiracy their comrades were in and thus are not the ones to share the blame.
Moral Event Horizon: If this wasn't already crossed with driving the entire Equestria — especially Celestia — into sadness with Luna's faked death, or the cruel treatment Luna receives from her imprisoners, then Navy Shield certainly does it by shooting and nearly killing Luna right after she's rescued and reunited with Celestia.