Every time, Arthur called Evelyn his wife, Regan had to control himself from getting down from his seat and beating him to death.
But he controlled himself. He knew that he couldn't mess up.
How could he not be aware of why Rex had been so worried! Rex thought that he would lose his calm in his anger but Regan wouldn't let that happen.
He was aware that this matter was complicated. They still didn't know exactly how powerful Arthur was but they could see that he was not powerless as well.
Regan was aware that he was supposed to handle this matter with his mind, not his strength. And hence, he couldn't let his anger interfere.
When Arthur finished, he calmly stood up and turned to look at his so-called father as he said