What is Miss. Wonderly afraid Thursby will do if he is confronted by Spade/Archer?

Miss. Wonderly is frighteded that Thursby will harm her sister. 

"Oh, but you must be careful!" Her voice shook a little, and her lips shaped the words with nervous jerkiness. "I'm deathly afraid of him, of what he might do. She's so young and his bringing her here from New York is such a serious-- Mightn't he--mightn't he do--something to her?"

"I do trust you," she said earnestly, "but I want you to know that he's a dangerous man. I honestly don't think he'd stop at anything. I don't believe he'd hesitate to--to kill Corinne if he thought it would save him. Mightn't he do that?"