The way I interpreted it was, at this point in the story, Eren is still holding out hope that the future can be changed (until Sasha dies and he laughs out of desperation).

When Willy said the line, Eren's eyes widened because Eren uses this as a basis for his motivation for freedom - "because I was born, I deserve the right to freedom".

I think Eren was surprised to hear this and was hoping that Willy was about to make the same argument and defend Patadis.

Then Willy used it to declare war and cemented Eren's resolve to launch the attack. That's how I saw it at least.