I was so focused on Thirteen giving the Skithra Queen her ultimatum then and then revoked that offer of peace to killing them and stating she’d ensure the Skithra won’t have a legacy left behind.
I forgot the Skithra queen’s specific threat: “Then we will kill all the teeming millions that infect this world, and you and your friends will die first. This is the choice you must make. Give us Tesla or the planet -and all who live on it- shall die.”
We’re not really focused on Thirteen at that point because of Tesla but as we learn later, merely saying ‘destroyed planet’ around Thirteen is enough to make her look like this:
In fact this was Thirteen’s expression during the threat:
So... at this point threaten Earth, basically.
This is what they’ll get, except now threaten the planet she’s adopted as her home and she’s not playing anymore.
She thought she was over her own trauma, but the Master opened it all fresh and new. And whenever the Doctor feels bleak and without hope... they hit the genocide option.