I'll admit I don't entirely understand the whole voltage/amperage thing.
Ok, here's the (oversimplified) way it works. Voltage is pressure. You need a certain amount of pressure to get through skin, clothing, etc. into the body. We'll say a good 300kV should do nicely most of the time. Then, you need current. Current is volume of flow. A taser or the like will only handle something like 3mA. We need a bare minimum of ~150mA, but something on the order of 3-5A would pretty well ensure a kill.
So, under ideal conditions (which vary more for electrical devices than for firearms), we need to push 300kV with enough hoss to handle up to 3A. That's roughly a megawatt. For reference, you can run a small locomotive on a megawatt.
All devices used to put electrical current into the body use tricks to improve conductivity, thereby dramatically reducing voltage requirements. Hence the blue goo on the defibrolator paddles. It helps the shock penetrate the very high resistance of the skin. If you were to design a taser with enough ballistic power to penetrate a couple inches into the body, preferably in the torso, you could probably get enough juice out of a car battery or three to do some real damage. 'Course, by that point, you've basically got an ordinary firearm with an unnecessarily complex electrical system.
