Putting an armed skymarshal onto the A/C is putting a weapon into the cabin were there was never one to begin with. The intentions are good but it's not the answer. Aircraft security on the ground is paramount. What goes into that aircraft (food carts,reading materials, people) must be checked, re-checked, tripl-checked and quad-riple checked. Passanger profiles at booking times must be thouroughly checked by federal or local law-enforcement agencies.
There is so much more we can do to prevent these types of accidents without introducing armed personel to the A/C cabin.