Witches Abroad/WMG

Lily's madness was due to her use of the mirrors

 * Like Voldemort's Horcruxes, Lily Weatherwax's power amplification of the mirrors sapped her sanity. Not to say that she was lovely before, of course. (Seeing as, among other things, her briefly-mentioned backstory hints that she was The Vamp who, unlike Nanny Ogg, used magic for her canoodling). But while the several passages mentioning the dangers of using two mirrors foreshadow her Karmic Not-Quite-Death, they also suggest spiritual mutilation that was never expressly elaborated on. This mutilation pushed her into Complete Monster status by taking away all her humanity, leaving only equal parts Tautological Templar and Super Villain.