The term bane was originally used to refer to certain types of plant. In the Middle Ages, a number of plants of the genus Aconitum were thought to have poisonous or prophylactic qualities, which would have dire consequences (Henbane) or liminal ones, like "Wolfsbane" or "Aconite." Aconite is thoroughly poisonous, but no one would bait a wolf-trap with it. So its qualities as a "bane" must be protective, for the wolf is more dangerous than a mere ravening carnivore.
Many medieval Europeans believed that they could become werewolves. Aconite was the toxic entheogen that could keep the dire transformation from happening. At a later date, milder poisoning was thought to trouble sheep in the U.S. southeast if they strayed into woodland and nibbled Kalmis latifolia.