Non leathal measures can be over come with simple measures such as newspapers under the shirt to defeat a stun gun. The only way to stop a threat is permanently. Aircraft remain the terrorists favoured target for the press coverage and potentially higher body count.
Air marshals make the call as to when and how to respond - no input required from the Captain and the marshals are responsible for the out come of their actions.
Hundreds of armed personnel (not just Air Marshals) fly every day without a single problem in the USA and I for one am glad for every single one of them that flies - not too mention the armed pilot programme in the USA.
Uk's murder rate is very similar to that of the USA despite the easy access to guns in the USA.
Finally to quote someone far smarter than me: when you outlaw guns only the outlaws will have guns.