The "fatality" in Avengers vs. X-Men is none other than the Good Shepard himself, Professor Charles Xavier. His death comes at the hands of his most prized student and adoptive son, Cyclops, who has been growing increasingly corrupt thanks to the dark influence of the Phoenix Force.
Just picture Patrick Stewart dying and I'm sure you'll be sad. It will be quite a feat if Cyclops comes out of this redeemed in any way.
Props to my buddyfor calling this a few months ago.