Lex had a tremendous amount of experience keeping his expression neutral, so even now, it wasn't really hard for him if he wished. But he did not wish. Of his sisters, Belle was the only one he hadn't been able to have a reunion with. She was the only one he hadn't brought to the Inn, so she was the only one whose difficulties he never got to learn about.
For the things that happened to Liz and Moon, Lex was mentally prepared. But this was the first time he was hearing about what happened with Belle, so his anger couldn't help but rise, which showed on his expression.
For the most part, his anger was aimed at Damian, yet both his parents did not escape being targeted by it either. If one was to go by their story, they were innocent bystanders, doing their best to survive persecution. To a large degree, that was also true.
But it was also that the priority from the very get go was the heritage, or whatever mission it gave to them.
