Bane (noun) is that which is destructive, causes great distress, annoyance or disharmony or is perceived as being \negative\" overall. Bane may also refer to death, a poison, a cause of ruin or woe. The origins of the word are closely linked with bone, wounds and death (especially by poison). The herb henbane is a good example, as it will poison your chickens if they eat it. Fleabane is another herbal example, a plant that was once used as a strewing herb (spread across the floor) to kill and prevent fleas.
Baneful (adjective) means something that is causing harm, ruin, woe, exasperation or death.
You will come across the terms bane and baneful a lot in Pagan and magical writing. Because there is not necessarily a perception of a good/evil or black/white dichotomy in the Craft. Magical practitioners use the terms to indicate anything that is negative or harmful and may be inclusive of several harmful factors including energy that is not conducive to the task at hand, even if it could be beneficial or neutral under other circumstances.
For example, an ice storm may seem baneful when it harms a tree, but may also be removing weak and diseased branches of that tree for overall survival of the species and the forest. This is why the terms bad, evil or negative are not used as often as bane, because it is a more specific term referring to how something exists within a paradigm, rather than just stating a blanketing fact."