There is no logic in this manga, so its possible.Son, that's some delusional type of shit.
What I think could happenNope, he will turn into END and Gray will be faced with a big choice that's debatable.
Will he kill natsu (a demon) like he promised his dad? Or let Natsu be a demon?
Sounds stupid but gray admitted that he couldn't kill his father even for the guild..but he probably would kill anyone for his father.