The way that I interpret it, OP’s girlfriend was simply being hyperbolic. I don’t think that she’s genuinely asking OP to kill her hypothetical kidnappers; specially given that, as OP mentioned, they live in a safe place.
Instead I think that she simply wants to be reassured that OP cares about her and her security. And then started playing around with the “but what about our cat?” thing because come on, if you’re thinking on outrageous scenarios, might as well think on them properly!