@Alletaire oh yea, I've caught up with the manga enough 2 know about that. (also a spoiler alert, ppl.) U have it mostly right, but he wasn't actually "created", so 2 speak. Natsu rly is Zeref's little brother, but he died. Originally, Zeref intended 2 revive him simply bc he loved him, but during his research he got cursed with immortality/contradiction as punishment 4 tampering with life & death. Afterwards, he continued his research 4 different reason - which is 2 destroy himself. He successfully revived Natsu as the demon END (Etherious Natsu Dragneel), creating other demons in the process including the Etherious.
@AtisutoMeru okay um well I'm not sure if I'm correct but I've heard (obvious spoiler alert stop reading now if you don't want to know k thanks) that Natsu was created by Zeref so that someone would actually be able to kill him. Zeref creates his underlings through the use of books, and Natsu's book is END or Etherious Natsu Dragneel. But uh... Yeah. That's what I think I know, someone correct me if I'm wrong.
@Alletaire Well, Natsu was Zeref's actual brother, but he died when he was young along with their parents. Zeref preserved Natsu's body to revive him again somehow. The sole purpose of Zeref's life was to make Natsu alive again. So, after being expelled from magic academy for studying black magic, he was cursed by Ankhseram & killed everyone around him with his newfound & unwanted Curse of Contradiction. The guilt ridden Zeref then started making etherious demons hoping that they would be able to kill him but none were able to do so. So, finally, Zeref created the ultimate demon END(Etherious Natsu Dragneel) with Natsu's body, thus fulfilling his dream to revive Natsu as well as creating an ultimate demon who would be able to kill him!!
Sorry, that was too long!!