I don't understand that rule. As there are almost no way to reduce POW to zero, killing a fetch would never happen in practice.
Besides, this is a rule from RQ2, but in RQ2 POW was a ressource being depleted by casting spells & spirit damage, hence the rule makes sense. But in RQG magic points play the role of a ressource, not POW.
So either this is a mistake in the RQ2 adaptation to RQG, and the correct rule would be to die at zero magic point not POW, or this is a correct rule that is intended to never apply. Don't know what to think.