Facts
1) The Larcade Dragneel said he is the son of Zeref and natsu is the son of Zeref (Latest chapter)
2) Mavis is the mother
3) Mavis kiss Zeref, caused Instant death of Mavis
4) Mavis and Zeref were close and Zeref liked Mavis (or I hope)
So my theory
Zeref made Larcade Dragneel like END...
