Rex tilts his head a little while still keeping his dark expression, he was truly on the verge of losing it on the inside but from the outside, he looks outright calm aside from his clear eyes filled with murderous intent.
Throughout his life, he always thought that watching his parents die in front of him was the worst.
It was a very painful moment in his life and even permanently traumatized him from the feeling of helplessness, he doesn't want to feel weak and helpless in a situation that really matters just like that particular night.
Now that he experienced this, he realized that he was completely wrong.
Although watching his parents die was something that many wouldn't be able to recover from, the fact that he has the absolute power to do something and make a change but was too late is a worse feeling than being helpless.
If he were here, nobody can't touch the strand of hair of his parents.