To keep things simple, actually. Essentially a "mook rule". Non-important characters/monsters die at zero hits.
And characters don't get "double hit points", they basically get Con stat +10 into the negative. (I need to add in that once they hit negatives, they will automatically lose 1 hit per round until death, keep forgetting to do that).
The foes die at zero, isn't really a different rule, but essentially just a short-cut for GMs so that they don't have to continuously track the death throes of foes, or such.
I will change the wording on that as well, so that it makes it the GM's choice to skip the death spiral......