The passengers on UA93 attacked the hijackers because they learned what happened to the other planes. The hijackers primary goal was thwarted as a result, likely saving the lives of thousands of people in the US Capitol building.
Passengers have intervened in several other terrorists attacks and now regularly intervene for other (non-terrorist) threat passengers.
It is extremely easy to get weapons into the boarding area, people do it accidentally every day all over the country and the TSA's own testing show that their screening misses the majority. Doors and procedures absolutely help as does the passenger response. Airport screening, OTOH, is primarily security theater.
Passengers have intervened in several other terrorists attacks and now regularly intervene for other (non-terrorist) threat passengers.
It is extremely easy to get weapons into the boarding area, people do it accidentally every day all over the country and the TSA's own testing show that their screening misses the majority. Doors and procedures absolutely help as does the passenger response. Airport screening, OTOH, is primarily security theater.