Here is the first of that relaying. Heather on mysapce pointed out this interesting article on Unplottable suggesting McGonnagal might be a DeathEater.

 The theory is long and well written. It suggests that McGonnagal is a sort of anti-Snape. Where he “acts bad but is in fact good” she acts like she is on the right team when she really isn’t.

 I don’t think it is going that way, but resist the impulse to write off any possibility. I put it in the range of 10:1-20:1. It would certainly be up to JKs standards to surprise us that way, after all this time. I think the moment in the books that seals it for me is during the astrology owl in book 5 when she wades into a hail of stunners (some coming from people we know to be Death Eaters) to help Hagrid. It is an interesting read.