Eren had come to realize something after he snapped out of his peculiar state induced by the sound-element arrow. He was going to be a liability if he doesn't fix the problem he has using the Hex spells.
And he had a feeling that even if he gains the needed control over his Hex spells without his dormant will taking over everything, he wouldn't be the same person he was back on Earth. Eventually, he would start becoming more of who he already was.
Something about him was bound to change as he kept on using the Hex spells. A tamed monster only became tamed after it had been put on a leash. But it won't change the monster itself.
Thus, as he grows and progresses further in his ranking journey, he won't work in the same capacity as the rest of the rankers in his guild. Plus, the Hex spells' usage was bound to spur his growth as a ranker. The resources he obtains through the guild won't be enough for him in the future.
