Based on Hajime Isayama's manga of the same name, Attack on Titan introduced audiences to the likable and seemingly typical shonen hero, Eren Yeager. Eren was presented as a freedom fighter driven by revenge, so many were surprised by his shocking turn from hero to villain when he instigated the Rumbling. Eren always had a longing for vengeance, having watched his mother be devoured by a Titan when Wall Maria broke. With an intense hatred for Titans, he vowed to destroy them all. But in an unexpected turn of events, the Titans were actually fellow Eldians who were forcibly turned into monstrous cannibals by a hateful world. This truth was so powerful and dark that it drove Eren off the deep end, and expanded the target of his fury to everyone and everything beyond Paraids' walls.
Source：https://www.cbr.com/aot-eren-rumbling-explanation/
