The cold tone from Kaito caused the man before him to hesitate. If it had been a few months before, Kaito would probably not have been overly influenced by the mention of his parents' death. In the past, the idea of parents was abstract.
He had managed to survive and thrive without their love or guidance. Since he was alright with how his life had turned out, he did not think it was necessary to care about the unknown people responsible for his existence.
However, his life in Mountain Ridge, the photos, the home videos and the stories from his grandmother showed him what he missed and made him feel cheated. Even in their death, they thought of him and tried to give him a better future by hiding their wealth.
As a result, he developed deep anger towards his uncle who caused those two people, who must have cared for him deeply, to disappear from the world.