Or, are you familiar with Chaos in Warhammer 40k?
Magic is a terrifying thing. He who fights monsters must be wary, lest he himself becomes a monster.
The horrible truth is that even the Inquistion, Ecclesiarchy, and Space Marines indirectly use chaos because they've de facto made the God Emperor a Chaos God.
Who's to say that the same principle can't be applied here? In order to destroy the witches, you must taint yourself by becoming a servant of a witch. But not just any servant, but the Queen herself. Because having a bunch of super-powered and uncontrollable psychic beings is a dangerous thing, and such power being used without regulation or control very well may mean the end of society. Especially if who becomes a witch is as arbitrary as it seems by the first chapter...
@Hollow7F