At some point in history Zeref witnessed many people dying in war. He also became cursed and helplessly started to take the souls of others without his own intention.
He is known to be from ancient times and presumably wields nearly unimaginable power, as even the strongest Mages shiver upon hearing his name. Ultear mentions that he alighted on an ancient land, mastered black Magic, gave birth to tens of thousands of demons, and threw the world into chaos.
Because of this, Zeref was sealed away for centuries. However, one day he apparently managed to set himself free despite the fact that Grimoire Heart did not collect all the keys to his prison, although the darkness inside him remains sealed. Apparently, the slumber made him a very kind person as he cares for others' safety now and wishes to be destroyed once and for all. At some time in the past, he apparently learned about Natsu and realized he could kill him someday. It can be presumed that he later wandered by himself, eventually finding Tenrou Island and deciding to stay there in seclusion, completely unaware that the island falls under the jurisdiction of the Fairy Tail Guild (and that no one else was...