You can’t get ‘electrocuted’ from an earphone battery, it’s not nearly powerful enough.

Well now, just to be contrary, I want to design a circuit that could deliver a deadly shock even with such a small battery.

I'm going to need a research lab, a few dozen live pigs, an assistant to do the soldering for me, and probably a lot of capacitors ... but I think it could be done. The key, I think, will be finding just the right combination of amperage, voltage, and frequency to best disrupt the rhythm of the heart.