User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-4933386-20150518131903/@comment-9595671-20150518132538

Natsu is Zeref's brother, but he died 400 years ago and Zeref tried in vain to revive him. In trying to do so he was cursed to be immortal and his magic became a form of death as punishment.

Eventually Zeref decided to find a way around the curse to kill him, the Tartarus demons were made to kill him but all failed. He decided then to make one last attempt and used his brother's body which he had been keeping in a tube so it wouldn't decay as the vessel for one final demon that could destroy him. That demon is END, and its why Natsu and END are the same.

In a sense, Zeref wants his brother to be the one to kill by having his strongest demon inhabit the body of his brother so instead of bringing him back he'll be with him in death.

So basically, Zeref created a new "soul" and then gave it life inside his brother's soulless body.