I feel I know what Sarah is doing. She probably could only pull off petrifying everyone, and is trying to force the princess to kill her so she could make her take back the royal sword. But damn.
It would also shift the entire blame to her, the princess wasn't able to save the people and protect the town because she wasn't strong enough or didn't care, she was tricked all along. Those who had no faith in her probably will now, when she kills her.