I'd go even further than Tony--Discworld Witches are anything but villainous--although Granny, for example, worries about going to the bad. The are the Guardians of the Borders, the wise women, herbalists, "doctors" and judges of Lancre (primarily). They do occasionally go to other parts of Discworld, but they (like the wizards) are generally concentrated in the Ramtops where the concentration of magic is highest. Their magic is quite different from wizard magic--but it has little similarity to the conventional notion of witches.
Witches can be opinionated, selfish, short tempered, and perfectly willing to trade on their position (eg. Nanny Ogg and her children, and daughters-in-law). They are not necessarily fun company, but they are who you want when things go "pear-shaped".
And Welcome to the forum--I envy you the chance to get better acquainted with the Discworld Witches (and the odd but nice lot who post here).