User blog comment:B214/Chapter 503: The Last Sight Seen/@comment-5303512-20160926122648/@comment-5303512-20160928142321

It doesn't matter if they are the same species, they know what to do, but their difference is their creation. Using Deliora is not a good example since back then Hiro didn't really thought of what to do with the demons.

Also, like I said above, Natsu was surprise to see Gray, that is all, that does not mean that would deviant from his current course of action, he will most likely just try to continue to go straight for Zeref while Gray will try to stop him.

Of course he wouldn't bother to finish Dimaria, because it serves no purpose or that he had no time to finish before the demon took over and left to go kill Zeref, otherwise he would have killed Dimaria less she would awake to kill Lucy and the fact that he didn't stay with Lucy to take care of her is another point to take as well.

Why would he be hateful and violent towards someone who failed to kill his friend. Hell even Future Rogue killed Future Lucy right in front of his eyes, even then he wasn't determined to kill, don't you see the parallels? The way Natsu is now, he is obviously showing some sort of resentment towards Zeref, perhaps some of his memories back then awakened too, maybe back then he hated Zeref for being created in the first place?