You don't think someone who takes a loaded gun onto a passenger aircraft is a bad guy?
Data point: I understand the difference between knives and guns, but still, having recently turned my house upside down looking for my Leatherman multi-tool (which includes a big knife), only to find it in the bottom of a bag that I had recently carried through security in a couple of different airports, I can say with certainty that someone who takes a knife onto a passenger aircraft is not necessarily a bad guy.