User blog comment:IamJakuhoRaikoben/Chapter 431 & 432: My sword is... & Brair in Love, Review./@comment-58.165.170.64-20150502041550/@comment-7418318-20150505071540

Because Mary's a psychopathic, evil bitch that loves to humiliate the foe before killing them.

And to give Wendy a chance to arrive in time to save Lucy. I doubt even Wendy could heal damage to the brain once it was done.