I usually think of the metaphor in terms of bringing a knife to a pillow fight as a play on "that's like bringing a knife to a gun fight" in which case my formulation is about having an innocent bit of fun and then someone going way overboard and doing something absolutely inappropriate. I sort of picture it in my head like everyone is having fun and then someone whips out a knife and then everyone starts to get very terrified.
I never really considered how a pillow would actually hold up against a knife in a fight. My money would still be on the knife, but I take your point.
