What are the two things that make Daniel think that Thacia is against him?
First, she acts rather snottily when Daniel seeks her help when he arrives at her home wounded. She is a relatively rich girl, the daughter of a distinguished Pharisean priest. She lives in a lavish home, attended to by slaves. The appearance of the lower-class Daniel, dirty, bloody, and in rags temporarily makes Thacia feel superior.
The second reason is not so much that she is acting "against" him but that Daniel sees no reason why he should really trust her. Thacia wants to take the vow of revenge with Daniel and her brother. The mere fact that she is female sets Daniel on edge. Given the submissive nature of the women in his own life and the patriarchal society in which they all live, Daniel finds little reason to think that a woman would be more than a hinderance to his goals.