Scarlett couldn't simply kill Theodor either because, in this case, he was innocent and had even tried to help her. It was all just a misunderstanding between them. However, she couldn't take anything for granted either. She needed something to ensure his silence, to prevent him from revealing her secret. That's why she mentioned the slow-acting poison, which was a fabrication. There was no such poison in her possession.
She had merely bluffed to dissuade him from exposing her identity to Xander. Furthermore, she needed Theodor because he possessed the connections that would pave the way for her plan to uncover traces of her mother's past in Country M.
...
"Theodor, regarding the man who beat you," Scarlett cleared her throat before continuing, "I hope you didn't hold a grudge against him. He is my older brother. He only confronted you because he was angry about how you had been pursuing me as if I were a criminal throughout the past year..." She explained.
