Fighting Annie Eren's fight against Annine in the first season of Attack on TItan leaves Stohess District in ruins. The assault eas ill-conceived,resulting in many civilian deaths and widespread destruction.
Losing Control During the Second Transformation During Eren's second transformation in Attack on titan,he loses control of himself and attacks Mikasa, one of his closest friends.Despit the mission's Success,his lack of prepartion and control nearly jeopardizes humanity's survival.
Using Falco as a pawn Eren disguises himself as an injured veteran and uses Falco Grice as a pawn to plan Paradis’ invasion and incapacitate Reiner. He ends up using tactics he previously detested and considers his actions with Falco to be one of his biggest failures.
Not Understanding Founding Titan's Power Earlier If Eren had known about the Founding Titan's powers earlier in Attack on Titan, many unnecessary and significant deaths could have been avoided. The ability to control Zeke's Titans could have saved an entire recruitment class of the Survey Corps and allowed Erwin to survive.
Succumbing to the Cycle of Hatred Eren's devotion to hating his enemies before understanding who they are causes him to join the cycle of hatred instead of trying to break it, which goes against a key theme of the story. His failure is his lack of desire to break the cycle.
Betraying his friends Eren’s reliance on his own special powers causes him to turn his back on his friends, brother, and followers in the final season, making the entire world his enemy and betraying those he wanted to protect. His failure is taking his self-reliance too far.
Killing Marleyan Civilians Eren’s indiscriminate killing of Marleyan civilians is his most inexcusable failure, as it renounces his humanity and embraces the role of a monster assigned to him. It shows that he has forgotten his roots.