(tl;dr version if you've somehow missed it: MZB's ex-husband was a convicted child molester, whom she aided and abetted, and her own daughter says MZB molested her too. And apparently all of fandom back in the day knew that MZB's then-husband, Walter Breen, was a jolly fellow fan whom you just shouldn't leave alone with kids...)
The thing is, this has been public knowledge for years. I first read this many years ago. With the way fandom nowadays hunts down every stray tweet and tumblr that someone might have posted in an ill-advised moment, it's kind of odd that somehow this is only becoming a Big Deal now.
I read The Mists of Avalon in high school and remember it being an okay retelling of the Arthurian legend, but OMG so men-are-slavering-beasts-women-are-the-s
So anyway, yeah, MZB was apparently an exceedingly creepy and horrible person, but like I said, until now I thought that was common knowledge.
And yeah, every single person back in the day who thought it would be too "mean" to exclude a known child molester from conventions because "Geek Unity!" deserves the beating they should have given Walter Breen.