The problem on American airlines is not "wacko" pilots. We vet our pilots extremely well. The problem is "wackos" in the passenger cabin. Therefore, armed pilots does nothing to increase the danger, and a great deal to decrease it.
Also, I agree with the prior poster who pointed out that a stun gun's elecrical charge could fry the brains of the airplane's controls. Remember the line from Apollo 13. "It's like trying to fly a toaster through a car wash."?
Congressman Billybob