Um, not quite. mA, milliamperes, is a rate of electron flow, or "electric current". mAH, milliampere-hours, is a measure of the total energy of a device, such as a battery. Battery chemistry is not a linear system, but for simplicity let's say it is. Then a 500mAH battery can supply a current of 500mA for one hour, or 1mA for 500 hours.
If a stun gun requires a 500mAH battery, this sounds like a statement about the energy needed for the circuit to function properly - translated to the world of battery ratings. By itself, it doesn't tell a complete story.
The current vs voltage thing is not as straightforward as we would like. In the classic analogy comparing electricity to water, water pressure is approximately equivalent to voltage, water flow equates to current, and a drop of water is the electron. Let's say you have a 500 horsepower water pump. You can funnel that energy into a very high pressure, high velocity jet about the diameter of a pencil, and use the water to cut granite, concrete, or lumber. It would slice through the human body like a band saw. Or, you can run the water through a 4" diameter fire hose. The stream won't cut anything, but it would knock you off your feet and blow you across the street. Same total energy, same medium, and lethal consequences in both cases, but for very different reasons.
Electricity can confuse the heart, re-synchronize it, stop it completely, restart it, fry the brain, relieve some brain disorders. There is no single, simple answer. But with a little analysis, some of the "contradictions" can be cleared up.
Take the stun gun. 50,000 volts. Sounds bad. Is bad. But (with the constant disclaimer that no two persons skin resistance or body chemistry are identical) any static electricity shock you can feel, such as the wool carpet/doorknob in winter, is usually above 20,000 volts. A "serious" static shock, the kind that make you say OW!, are in the 100KV range. So why do stun guns stun? Because they have more celectrons available to flow - more current.
When nothing is touching the stun gun, it arcs very nicely and everyone goes ooooh. But the gun does NOT make 50KV when it is touching you. It can't. If the typical skin resistance is 5K, and the voltage is *sustained* at 50KV during contact, that's a current flow of 10 amps, or 500,000 watts. Ain't no hand-held battery gonna be deliverin them. What actually happens is that the body "short's out" the gun output. Not a dead short, but close. The 50KV actually changes the body chemistry in the area around the contact points, so precise skin resistance measurements are difficult, and don't relate to "normal" skin. But the voltage across the two contact points drops dramatically upon contact, limiting the total energy delivered to the body to non-lethal levels.
Without getting into overload situations, we expect a battery to deliver the rated voltage under both light and heavy loads. This is called a constant-voltage source. Within the output current design range, the output voltage remains constant. 9V at 1mA, 9V at 10mA (but for less time), etc. But a stun gun output behaves very much like a constant-current source. It delivers a few mA at 50KV when there is no contact, or about the same current at a way lower voltage during contact. If I remember correctly, the voltage uses for executions is about 2KV, but with the current limit at a much higher value.
Back in my TV days I became acquainted with 900V boosted B+ in a color TV. It drilled a small hole through my thumb. Self-cauterized, zero blood, and one of the truly memorable pain experiences in my life. Occasional contact with the 20KV on the picture tube was an occupational hazard, but totally survivable. And yet, contact with 220VAC on apower line can be instantly fatal. Try this as a summary:
High voltage with very limited current can be painful, but not fatal - not enough current. Low voltage with lots of current can be unnoticed - people handle car batteries every day without any protection - hundreds of amps, but not enough push. But between the two is a very large number of voltage/current combinations, any of which can maim, kill, or cure. The current does the work, but only if the voltage is high enough.