User blog comment:IamJakuhoRaikoben/Chapter 450: Alone in all the World, Review./@comment-5896530-20150831183717/@comment-14.100.132.21-20150901070454

^this And to add another point, from his childhood(or not) basically everyone from his Village just die, and his brother is gone, he haven't cared for anyone in a long time. Which is why, when Zeref said "I never loved anybody so much" to Mavis, his love might not be too serious, it might not even be love if it were to compare to others, but to Zeref, that was the strongest feeling he had ever felt for others. Thus, it was strong enough to kill Mavis, and not the other way round. (Thats my opinion though )

I do hope that next chapter there will be more explanation on that part though.