I've never heard anything about witches being responsible for undead mobs. Are you just making up a rumor so people will support your idea?
Perhaps I should rephrase. A lot of people are *speculating* that witches are responsible for undead mobs. I don't think Mojang will try to incorporate this into some sort of plotline, nor should they even try. But take a look in the many threads about how people want to "fix" witches and many posters there have stated this is the story that comes up in their imaginations.