Why not just use a gun if you're going to kill the person? At least with a gun the person could still survive and you won't go to prison for a long time. There is a thing called "excessive force", that is unless you live in Texas...
Possession of a gun is a SERIOUS offence in the UK, even the Olympic handgun team aren't allowed to own or use handguns. Gun clubs are allowed to have rifles, but it's VERY seriously administered. The only handguns allowed are ball and powder types, nothing you can load in less than half a week!
However, possession of a stungun is also a serious offence, as is gas or pepper spray.
However, as a martial artist, any attack on your person should be considered life threatening, and your response should be swift and brutal - and completely automatic - it's easier to argue in court later, than on a slab in the morgue.