|I'm not bad, I'm just...actually, I am.|
If a witch (hags are the closest that AD&D gets to traditional witches) sets up home hear a stream then you can be sure that the stream becomes filled with slimes and its banks become marshy and unwholesome; the local fauna becomes less and less colourful and fungus will spread. This is not a specific power of the witch, basilisk, or dragon - it is the effect of their Evil nature.
Evil in this sort of fantasy is a corrupting force which can alter nature, causing malformed offspring such as giant versions of normal insects, spiders, frogs, and so forth.
|You can still tell it's a wig|
One aspect of this "fairy-tale ecology" that doesn't go down well in our post-modern world is the reverse of the coin: that Good is normal and normal is Good; indeed that beauty is Good. There is certainly a strong strand of this in folklore and the attitude often leads to real-world racism and xenophobia when the other party does not fit local ideals of beauty (hence all the Nazi head-measuring crap, for example). But over the centuries the inventors of these stories have seen the dramatic possibilities of subverting the trope - of making the beautiful step-mother be, in fact, Evil - that to a degree that has to a degree reversed the notion, so that a person who is too good-looking seems suspicious.
|"You look shocking, my dear.|
Shocking, geddit? Oh, please yourself."
So there's a double standard here: we very rarely see honourable and upright heroes and heroines who are not good-looking and physically idealized before the 1960's (when the potential for ugly central characters to be "pure of heart" instead really took off), but the beautiful can be Evil.
In AD&D we can see the influence of this pre-modern attitude by a quick scan of the Charisma table on page 13: assassins are the only class with the possibility of Charisma below 6, paladins have to have a charisma of 17, and humans view half-orcs as never more than 12 CHA, reflecting the xenophobia aspect (not quite sure about the druid's place in this).
|Tell-tale sign of Evil taint:|
two right hands
When UA added comeliness, the modifiers involved meant that assassins really are uglier than average, and paladins more physically attractive on average. The comeliness rules even have a half-arsed attempt at suggesting that the sufficiently hideous is in fact attractive to those who are Evil. The assumption is that Evil tends towards ugliness and Good towards beauty. They are tangible forces that have effects on living things.
This all links to the central notion of D&D being about archetypes (classes) rather than the much more nuanced character creation of RuneQuest and its descendants (although, of course, there are very few physically attractive Mythos creatures in Call of Cthuthlu). For D&D, simulation is still a key to the rules - they're not intended to be abstract - but it's always good to remember that it's not a simulation of the real world.