Actually, you can kill yourself with a single 9 V battery -- or the 12 V battery of your car. One man did:
The computation goes as follows. The issue, as several people have pointed out, is that it is current across the heart that causes defibrillation (basically interrupting the heart's natural rhythm so that it pulses chaotically), not a matter of cooking the person (which will also work, BTW, but isn't the most common cause of electrical shock deaths). It isn't even the case that more current is always worse -- there appears to be a range of currents that are more toxic than others. A brief explanation of this is here:
The maximally toxic range of currents across the thorax is empirically 0.1 to 0.2 amps. Below that it isn't enough to defibrillate, above that the heart muscle clamps all the way which means that when the current is removed it is actually more likely that it can with help or will on its own restore a normal rhythm.
The internal resistance of the human body once you introduce probes through the comparatively insulating skin is around 100 ohms. A 9V battery across ~100 ohms makes a thoracic current of roughly 0.1 amp, right at the start of the maximally fatal range. The Darwin above was given because an idiot didn't believe this and stuck probes through his skin to "prove" that it wasn't so.
Personally I've experienced shocks from 12 V car batteries when screwing around with them on rainy nights with salt water on my hands. That's another good way of reducing skin resistance. I didn't take the hit across the torso, but it was every bit as painful as a 110V shock through dry skin -- more so, actually -- and caused my muscles to contract like lightning.
None of this is actually news -- it has been known as long as there has been electricity, because people have been killing themselves accidentally with electricity just that long. My scout leader 50 years ago worked for GE (as an inventor, actually -- one of the people who invented the photodiode controlled light). He taught me that long ago to ground one finger and then brush another finger of the same hand against any possible hot wire so that you find out with a jolt across your hand, not through your torso. Hand to foot, hand to hand, not so good. People used to kill themselves all the time touching hot electrical switches while standing in wet feet on bathroom floors before ground fault circuits were invented and mandated by code.
None of which has much to do with TFA, but it is good to know if you work at all with electricity. Physicists need to know it just to be able to teach it to their students so THEY don't kill themselves accidentally one day. It isn't the voltage that kills you, it's the current, and it doesn't take much current to do the job (or much voltage to create a fatal current).