User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-35340256-20150831100207/@comment-7418318-20150831205019

I think Zeref didn't actually glare at Mavis. Apart from him being unable to actually see her, his comments also made me believe the glare was meant for Mankind as a whole, not for Mavis.

As for Mavis... he did kill her and vowed to kill everyone else alive, which includes people Mavis cares grately for. And he's also indirectly responsible for first getting her cursed, putting her into the position where she could die in his arms in the first place.

I think she has plenty of reason to glare at him.