William's main goal from killing lots of her masters today was to make Sara's mother unable to think of attacking him seriously for revenge. Like this, she'd be forced to accept her fate.
Even if she thought about asking for help from the other side of the portal, she'd rethink this move hundred times before doing it.
When she'd see the crystal, realise that he recorded her words and plans towards the head of her palace, things would turn ugly for her.
Her palace wasn't a merciful place and wouldn't tolerate such treason in their eyes. It was normal to have dirty aspirations and thoughts, but to speak about them in front of an outsider, in such a way, and get recorded on top of that was a crime in itself.
William knew the palace head would get mad at Sara's mother for getting tricked like this and get herself recorded on a crystal. It was obvious she fell for a trick to get recorded like this. And that meant she was acting sloppy.