Witches, maidens, it’s all a matter of perspective really. At the end of Peredur, the hero comes upon nine witches. Luckily Gwalchmei shows up with Arthur’s warriors and together they make short work of the women and their cauldron. Part of the back story is that the witches had killed his cousin, making their deaths a matter of revenge – standard legal practice in the Middle Ages.
Magic and women are actually common even in the earliest Arthurian pieces of literature. In Preiddeu Annwn, Arthur is credited with stealing a magic cauldron from nine maidens – without the mention of revenge. It’s a theft, like a cattle raid. Now, I realize that doesn’t sound nearly as noble as revenge, but it is the earlier work, both linguistically and culturally, and the simple fact that it doesn’t sound “Arthurian” is a good reason for believing that version. When you think about it, even without the motivation it seems more wrong to be stealing (and presumably killing) from “maidens” than “witches”.
Safe to say that Preiddeu Annwn is the older and more accurate version, which means we need to reconcile a British chieftain with stealing kitchenware from a group of defenseless women. That seems a little ridiculous though. Not even in the most gruesome tales do kings do things like that without some reason.
So maybe the reason is in that cauldron. The Welsh would have called it a dysgyl (translated to French as cors or corn as in Carbonek), and in the hands of women and certain very lucky kings, cauldrons had the power to revive the wounded, give life to the dead, and act as a cornucopia. The cauldron was a central element of many fertility cults involving Belatacudros. We also know that, as Christianity took root in Post-Roman Britain, pagan groups were often attacked with their temples destroyed and followers often killed. Now Arthur was probably not in regular contact with the continent, but there was trade up and down the western coast with Europe and he may have had a port. Besides, even without the push, uniting his kingdom against a group that was different might have proven very useful if he had tried to spread his power base much beyond his hall and some local villages.
Peredur finding and helping to destroy a coven that had killed his cousin? More like Arthur leading a crusade to gain political support.