From what I can recall, Cass only says the whole "I'll maim you." line only twice, once when her and MC reunite for the first time when his exile/banishment ends, and second time when MC was watching Aker and his uncle spar.
But I found the instances of when she says that to be playful, meant to be more of a test towards the MC (and also the player

) to see whether they take them seriously or not.
Similar effect to Luna being "Didn't I kill you before." or other comments that some people playing that game found offputting; though I had no issue with them at all.
I think
Lazy_Monkey will really develop Cass's character so there's a lot more to her than what is seen on the surface level (classic case of "Don't judge a book by its cover.") like HopesGaming did with Luna for DeLuca, especially if the player is on Cass's path.
But I guess time will tell.