Considered Fairy Tail's number one enemy, Zeref is known as the Black Mage. His use of dark magic and command of the Alvarez Empire's Spriggan 12 make him a terrifying foe. The mention of his name alone makes anyone tremble in fear.
Not many in the series knows Zeref's background, but he arguably has one of the saddest backstories of the entire series. His character is misunderstood since his story centers around him wanting to reunite with his brother and lover.
He Lost His Entire Family To Dragons
Zeref was born 400 years before the start of the story during the time of the dragons' reign. Dragons ruled the land and easily overpowered humans. An unnamed dragon raided the village where Zeref lived with his family and killed everyone. He survived, but his parents and younger brother, Natsu, died in the attack. Now orphaned and alone, a young Zeref was left to fend for himself.
A common trope in anime and manga is for a character to lose their entire family or close friends. Attack on Titan's Eren lost his mother and many of his comrades to titans. The Elric brothers lost their mother to illness. Tokyo Ghoul's Hinamai watched her mother get killed by ghoul investigators. The tragedy throws the character into a state of despair and sadness and causes them to, at times, act irrationally. Zeref justifiably felt upset about his loss and only wanted to be reunited with his family.
His Curse Kills Anyone He Loves
While studying life and death to find a way to bring Natsu back to life, Zeref became cursed with Ankhersam's Curse of Contradiction. The nature of the curse caused Zeref to kill all life around him. The curse additionally caused the victim to become immortal so that they suffer the effects forever. To limit the curse, the victim had to hate and disregard life itself. Zeref loved life, but the more he loved, the more he unintentionally killed.
The immortality drove Zeref mad. He wished to die, so he created the Etherious to try to kill him. When he met Mavis, and she became afflicted with the same curse, Zeref found a sense of belonging with her. Someone else finally understood the pain he was going through, and he fell in love with her. However, his curse ultimately took Mavis's life away, and Zeref was left alone again.
Natsu Views Him As An Enemy
Zeref suffered for centuries to bring Natsu back to life. When he finally succeeded, Natsu didn't remember who he was. As seen during the Tenrou Island arc, when Natsu and his friends ran into Zeref on the island, Natsu didn't recognize his brother and punched him. Zeref was unfazed by the attack but cried because of how the world continued to reject him. Zeref remained lonely and ostracized because of his curse, so he began to hate the world instead.
Later in the Alvarez Empire arc, the brothers were on opposite sides of the war. Zeref was an enemy to Fairy Tail that Natsu had to defeat. His attack on the continent of Ishgar was part of his plan to provoke Natsu into wanting to kill him. He ultimately failed and died with Mavis. The two are consumed in light and disappear, leaving behind Zeref's pendant with a picture of him and Natsu inside.
Before dying, Zeref did express his regrets for not being better to Natsu. He wished he could've made it up to him. Despite being known as the Black Mage who created evil demons, deep down, he was only an older brother who loved his family. His actions were cruel, but he did what he thought was the best way to be reunited with the ones he loved and put an end to his enteral suffering.