Well,if you look at the end of the spear,her skirt,and her hair,she seems to be disintigrating,so my guess would be killing Oktavia send her over the edge,thusly prompting her own witchification.
Well, since her own Soul Gem seems to be missing from this picture (the empty space of skin on her chest is where it should be) I'd guess that in this picture, Kyouko is holding her own Grief Seed.