Yeah, and really despite the anger and tension they sometimes have with each other, they aren't like sworn enemies or anything. Cassandra can be harsh because that's her job and her life, but to be fair the matter she pressed him with, including threats and violence, I don't blame him for how he reacted. Some stranger kidnapping him and suddenly asking him all these questions about someone he cares about, all while making it sound like they are a target and a bad person - he was protecting his friend. And on Cassandra's side, she was desperate to try to fix everything and this way was how she knew to get things done because that's what she's always done. Both of them had legit reasons to act the way they did and you can put blame on them both equally for perhaps not going about it the right way, but their intentions were in the right place.
Yeah, they both had good reason to do what they did but naturally they are going to start off on the wrong foot with each other because of that.
They have some hilarious dialogues and banters together:
Cassandra: Have you heard from any of your Kirkwall associates Varric?
Varric: You're asking me? So you don't read my letters?
Cassandra: You're no longer my prisoner. Much as you like to act like it.
Varric: Yet I still get all the suspicion.
Cassandra: I am not without sympathy. Especially given recent events.
Varric: Why Seeker I will never accuse you of having sympathy! By the way I tend to refer to my "associates" as friends. Maybe you're not familiar with the concept.
Cassandra: (sigh)
Varric: Think you'll ever go back to Nevarra, Seeker?
Cassandra: Why? Are you eager to see me go?
Varric: I wasn't, actually. But, now that you mentioned it...
Cassandra: How do you know I wouldn't just drag you along?
Varric: Be still my heart. I've grown on you.
Cassandra: Like fungus.
Most of the time they honestly seem to be more teasing each other than seriously angry with each other. It is pretty tame compared to some of the banter between the DA2 companions.