User blog comment:Royice/Theory on how Zeref know Natsu/@comment-24066070-20140113022509/@comment-7418318-20140113081220

Considering that Natsu described (and the flashbacks showed) Igneel as a loving parent, I somehow doubt it.

I guess it is possible that Natsu simply grew on Igneel over time, causing his behaviour to change, but still I don't think so.