Yeah, a lot of people seem to throw hate on Bianca simply because of that one line, which is a shame because until then she's an awesome character who is thoroughly enjoyable to watch? I think that was just a bad line in how it was written and presented, which spoiled the scene for everyone?
I think that was intended to be a friendly mock-threat that you'd see between two people comfortable joking with each other, but failed because she'd not had enough interaction with the Inquisitor to allow for that level of familiarity, unlike the playful banter between her and Varric?
Give that line to Scout Harding for instance, no-one would likely have an issue because we've had enough time to know her properly and that line would probably fit with her sense of humour and how she's a adorable bundle of murder? But if she'd said something like that in our first conversation with her, we'd probably be a lot less fond of her character and think she's a bit psychotic? Especially after seeing that little smirk she does in the cutscene after icing that mage and templar?
Yes, I think it was a fumble on the writers' part because she does seem crazy for delivering that line to someone as influential, and crucial to the task of saving the world as the Inquisitor is. Especially if they not only forgave her mistake but defended her.
I know it might seem like some of us are just getting our hackles up about being threatened at all, albeit jokingly (probably) but I don't think that's the entirety of it for most people. It seems to me that any positive characterization she receives before that line is overshadowed by how bratty, foolish, and just plain out of place it seems. The fact that you have no option to react to it is part of why people accuse her of being over favored by the writers.
While my first impulse isn't to hold her accountable for making an honest mistake, I understand why a lot of people take issue with not having that option available to us. In the same breath, I also understand why the writers wouldn't want to tackle that mess of a plot derail, because then they'd have to write Varric's reaction, and that could get chaotic.