It's worth noting that Pratchett's tongue-in-cheek etymology of "wizard" from "wise-arse" was actually not far off the mark. While the wiz- part does mean 'wise', the -ard suffix is derogatory. Think "bastard", "drunkard", and "coward".
Having done some digging, I can't actually find support for the
English interpretation of "-ard" as a negative in itself. Originally it referred to "hale and hearty," and was primarily present in Old French names (Barnard, Gerard, Goddard); when adopted by Middle English, it became an
intensifier of the previous element. So yeah, when the initial element is bad--illegitimate, inebriated, quailing--it signifies something bad. When the initial element is neutral, it acts more like an "actor" suffix--e.g.
buzzard. For a positive initial element, like "wise," it pretty clearly means something good. The commonly negative uses in general nouns were retained due to the influence of other Germanic languages (Middle High German and Dutch), where it
was used as a negative; the commonly positive uses now really only linger in names.
Also, going back to Old English, wicce and wicca are, respectively, the feminine and masculine forms of the same word, what I'd translate as "witch" and "warlock". (Yes, Dianic Wiccans, sorry, a wicca is a man. Also, you're pronouncing it wrong.) Our linguistic forbears do not seem to have been discriminatory in their disdain for black magicians.
This, at least, I can say is perfectly true. "Wicc-," which would be inflected for gender just as you've said, referred to a particular kind of practiced magic--its origins are obscure and difficult to pin down. However, given its usage in glossaries between English and Latin, it seems to have referred to relatively "dark" practices--at least from what I can get on Dictionary.com. Certainly, it's also the root of the modern word
wicked.