Eren-san asked to be de-infiltrated if he was separated from his friends, so that he could try to join them. It's not a choice I would have made, but it wasn't my choice to make, and I couldn't in good conscience withhold his ability to make it for himself. [Eren has in fact thanked Naoki for doing that, since then.]

Armin-san got upset when I wouldn't back down from that because he didn't want his friend in danger. After admitting he knows Eren-san well enough to know he would have wanted the chance to try to get there, even if he failed.