Re: THC, the active ingredient
Bathtub gin killed because it was contaminated with things like solvents and methanol
It was mainly the methanol. All fermentation produces methanol as well as ethanol. It is not the methanol itself that is particularly toxic, it is the breakdown products when the body disposes of it. Too high a concentration of those breakdown products causes the problem. Fortuitously the same breakdown mechanisms tackle both methanol and ethanol, and the relative concentrations are such that dealing with the ethanol slows down the rate at which methanol is broken down, to the point where the body can deal with methanol's breakdown products fast enough that they don't cause problems.
Those relative concentrations can change during distillation, because methanol boils off first. Do a good job (use a thermometer and discard the low boiling-temp fraction) and the resulting spirits have an even higher ethanol/methanol ratio. Do a sloppy job (where you don't bother bleeding off the methanol) and you leave the relative concentrations the same, as long as you dump the entire output into a single container before bottling and it's still safe (well, as safe as alcohol ever is).
Do a really bad job, where you don't take steps to discard the methanol and where you put small bottles to the outlet, one at a time, and the first bottle can have a very high ratio of methanol to ethanol. Maybe the first few bottles. Or, if your still is big enough, the first dozen bottles. And that's where the problem lies. Some bottles in your run will be relatively lower in methanol than the feedstock and some will be higher.
Note that it is illegal to distil alcohol unless you have the relevant licence, so don't try this at home. But if you do, make sure you do it right.