The Gauss's law argument is as follows:
1) We know that there cannot be an E field inside the conductor, because if there was a net E field inside the conductor, then it would move charges, and the staticity assumption would break.
2) Now, assume that, in some region of the conductor, we have a net charge accumulated in some region.
3) Then, we can enclose that net charge in a Gaussian surface, and necessarily, it will have to obey $\oint {\vec E}\cdot {\vec dA} = q/\epsilon_{0}$. Since the RHS is nonzero, the LHS has to be nonzero, therefore, we have a net E field. We have arrived at a contradiction, so therefore, our assumptiont hat we can accumulate a net charge in the interior of the conductor must be false.
Which electrons kill you during electrocution ?
None of them. As you say, there are already plenty of electrons and ions moving around in your body anyway, so adding a few more makes no difference.
Death from electric shock is usually caused by the electric field, which disrupts the nerve signals which control the synchronised beating of the heart muscle. This causes ventricular fibrillation which prevents the heart from pumping blood around the body and leads to death within a few minutes if not treated.
And the usual treatment for ventricular fibrillation is to administer another carefully controlled electric shock from a defibrillator, which (bizarrely) disrupts the heart's rhythm even more severely, but then allows the body's natural pacemaker to re-establish the normal rhythm (if the patient is fortunate).
A very high voltage electric shock can have other serious effects - it can actually stop the heart completely or it can cause internal and external burns.
On the other hand, an electric shock that does not pass through the heart may have no ill effects at all. And the administration of electric shocks in a range of medical treatments called electrotherapy may even have benefits for certain health conditions.
Best Answer
The switch really has 2 positions: on and off.
However, when you move the switch very slowly, it may leave the closed position slowly. When the switch is just barely open, the field may cause the air to break down and start conducting, to form a spark (as @anna v explained). To rephrase, the reason why sparks happen is because the switch may only be open a tiny amount, not enough to stop current from flowing through the air. If the gap then increases further, the spark may persist because the air is now acting like a conductor rather than an insulator.
Switches are usually designed to prevent this from happening. They have built-in springs that act to open the contacts quickly and completely, thus preventing sparks. However, with many switches, moving the toggle very slowly may cause the contacts to separate a tiny bit, before they fly completely apart. Older designs are likely to suffer more from this.
Switch design is easier for low-voltage switches, because high voltages are more likely to cause the air to break down and cause a spark. It is the voltage that causes electrons to jump across the gap and create the spark. For that reason, high voltage switches are also larger: they have to be large enough to keep the contacts far enough apart when the switch is open. Remember that high enough voltages can cause electrons to jump between clouds and the ground - that's called lightning.