Man, all I see here is the makings of Electricity 101. [;)]
What kills is current, or amperage, which is the number of electrons flowing through the body. A very tiny number of electrons flowing through the body is harmless. A shitload does bad things, like put muscles into tetany and barbeque the flesh.
The body has an intrinsic resistance, measured in ohms, that can vary because of physiological factors and the path of measurement. As the name implies, the higher the resistance, the harder it is to push electrons through the body.
Voltage is pressure. If the resistance is high, a high voltage is required to push a given number of electrons through the body. Cut the resistance in half, and only half as much pressure, i.e., voltage, is required to make the same current flow.
One more viaiable in all of this is available source current. A car battery provides 12 volts of pressure, but has enough electrons stored in it to briefly supply a huge amount of current, maybe a 100 amps (not real sure of exact figure). I can also hook up 8 AA cells in series and also get a 12 volt supply, but it can only provide a tiny fraction of the current available from a car battery,
So, a 50,000 volt Van de Graaff generator can supply only a milliamp (again, not sure of exact numbers), and this is not enough current to be dangerous despite the fact that it is being pushed by 50,000 volts. You'll feel it, though. [:)]
Most household circuits can supply 15 or 20 amps, as you know, at 120 VAC. However, the resistance of the human body tends to be high. Turn on an ohmmeter and grab a probe with each hand and it will probably read 100,000+ ohms. The actual current that flows depends on your resistance, and the path through the body. Obviously, if you touch a live wire with you hands and are grounded by your bare feet, the current will travel the length of your body and go through internal organs in the process. Not good.
Obviously, not pulling the breaker before working on a circuit is little different than trying to clean a firearm while it is still loaded. Both can get you removed from the gene pool in short order.
I hope you did not find my little expose shocking [shock] [:D]