User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-26138717-20150831092542/@comment-7418318-20150831204803

There can be many explanations why it was Mavis who died.

Of course it's possible that Zeref loved her more than she did him, but then maybe it's the other way round. If love really is the One Magic, Mavis' love might have been strong enough to dispell the curse on herself. And the moment it was gone, so was her protection against Zeref's life-leech.

It could have been another intervention by a very VERY spiteful Ankhseram, who believed Zeref didn't deserve love.

It might have been that the curse was stronger in Zeref due to being cursed directly by a god, rather than accidentally by miscasting a spell.

It might be Mavis poor physical condition.

It might have been that due to Zeref's phsychological instability or lesser appreciation of life.

It might even have been due to Zeref having more magic power and thus radiation stronger death-waves than Mavis.

And those are just the ones I can come up out of the top of my head without thinking. I'm sure I could come up with more if I put some effort in it.