How to connect multiple capacitors in a series?
You basically got everything wrong ;)

These USB-Killer doohickeys use a DC-DC converter to step up the USB 5V to a much higher voltage, something like 100 volts or more.

Like in a photoflash, this high voltage generator charges a capacitor bank. Since caps are readily available at these voltages, there is no need to use a series combination. Several ceramic caps are wired in parallel simply to stuff more capacitance in the height footprint of a USB key. A single electrolytic cap would work just as well, but it would be larger, and thus unable to masquerade as a USB key.

When the caps are charged to the target voltage, a switch (most likely MOSFET or SCR) triggers and discharges the caps back into the USB port, destroying it.

Pretty idiotic... I mean, you can give such gadgets to people and laugh when they zap their computers, but then this raises other philosophical questions, like, do they own a baseball bat or a crowbar, and what will they do with it once they get really pissed?.......

EDIT: about the nature of the voltage booster

Charge pump? Probably not. A charge pump is a simple way to double a voltage, or convert a positive voltage into the same negative voltage, but if you need a high output/input voltage ratio, as is the case here, the circuit becomes large and complex. An advantage of using a charge pump voltage multiplier for high ratios is that the components don't need to be rated to the full output voltage, good if you need 100 kV, but not here.

Photoflash charger? Large step-up ratios are most efficiently done using a transformer, so this would be the best solution, but since the point here is not efficiency or conserving energy, the extra complexity and cost would be unwarranted.

Since they mention using a negative high voltage (in order to make it simpler to switch with a big fat NMOS then the most likely candidate would be the inverting buck-boost converter. It is simpler, uses one switch and one diode, and one inductor, plus it is cheap.