His own free will

When Eren still lived shiganshina, he grew an infatuation with the scouts. He would eat his dinner on his house's steps to watch the scouts leave the cities walls, paying extra attention to the Captain who was straddled on a horse that towered over all the other horses, and whose pitch black coat matched his rider's hair, who always kept the same stern but menacing look when he gave Eren a sideways glance.
The memory of the captain was enough fire to keep his passion, the memories replaying in his head during his teen years, his training and even when said Captain kicked the shit out of him in a courtroom and threatened to kill him.
But now, sitting in that dining room, all Eren wanted to do is carve the Captains teasing eyes out of his skull to stop them from baring into his soul.

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