I explained two already. The one regarding slippery slope is where you put forward all the bad consequences without specifying them and stating realistic potentials. Another was your branding the proposal as simplistic which is an example of complex question.
You might find this site interesting
http://www.datanation.com/fallacies/index.htm
You may not be a liberal but your tendency toward arguing based on logical fallacies makes you prone to the disease.
Why are you afraid of responsible Americans defending themselves?
We ahve shown that the cockpit doors ARE NOT on present aircraft, don't work if they were, and can't be installed, nor can they be left shut during flight.
We have shown that ANY air marshall can be immediately killed silently and quickly ... if by nothing else than a plastic stick THROUGH the seat behind him right into his gut.
THAT gives the hijacker IMMEDIATE and UNPREVENTABLE access to the MARSHALL's gun ... WHILE LEAVING THE PILOT (the ONLY person who needs a weapon!) STILL disarmed. Against the NOW ARMED attackers.