Eren had already started to get up when Cairbre emphasizes that it likes the reasoning as much Eren himself does - and possibly even less. He pauses for just a moment, but then decides to move to sit more comfortably on the tire rather than try and stand again. The young man still isn't calm, not by any stretch of the imagination, but his rage is no longer flaring quite so violently.

Now that he's thinking a little more clearly (and Cairbre has stressed the point twice) Eren finally stops to process the implications of what it has said. "...What do you mean 'not again?' Is that what happened to the friends you're trying to bring back? They were killed for breaking with tradition!?"

Eren wouldn't put it past the Tower or especially the Devil to kill their own over something so trivial.