Cassie actually faced a recent situation where a dear friend of hers, who is a synth from an alternate universe, was in a position where if she learned someone from her past was on Primal Earth, her hard-won freedom would have gone poof. The man was a monster, and she and one of her best friends (one of her self-adopted sisters) were "on point" to find the guy and make him cough up the codes they needed so that her friend would be free.
The problem was that he was too dangerous to be left free....her sister was willing, if it came to it, to kill him. Cassie faced the possibility of doing it herself, despite her own vow to never kill, for the purpose of sparing her sister that burden.
As it turned out, they found a solution (friends of her aunt with a space station imprisoned the monster in the form of a man there) without killing, but she was ready. Because she'll do anything for her framily, no matter the cost.
This follows her sister, Jean, herself having her mind shattered last month when "I'm amnesiac, I want to know about who I was" suddenly became "no, you REALLY REALLY DIDN'T"...