No one else cried, because no one else had to tell their father that they could not not save their loved ones. No one else cried because no one else even TRIED to pull the trigger, apart from Namor of course.
And he couldn't say his stance with confidence, because deep down he KNEW his father was right. That's why they mind wiped Cap in the first place. Because the Illuminati knew blowing up planets was necessary. It was what was needed to be done... he just couldn't bring himself to do it. And his family and loved ones were about to die because of that inability.
It is a COMPLETELY valid reason for him or anyone else to break down. He is human, believe it or not. An like all humans even he has limits. If this isn't a time where he can break down, really what is?