In destroying Old Misery's house, Trevor is not fulfilling some secret vendetta against the old gentleman. He does not dislike Mr. Thomas, nor does he hold a grudge against the older man. Trevor's act of destruction was based primarily on the allure of the challenge. He planned with the other boys to raze the house to see simply if they could pull off the stunt.
After the boys break into the house, Trevor makes a discovery: Old Misery's saving of banknotes tucked into a mattress. When he announces his plans to burn them, Blackie is unsure of T's motive:
"You hate him a lot?" Blackie asked.
"Of course I don't hate him," T said. "There'd be no fun if I hated him."
Trevor does not hate Mr. Thomas but does perhaps act out of dislike for what Mr. Thomas' house stands for: tradition, hierarchy, a higher social class. Trevor's act of leading the boys to destroy the grand old house suggests the boy's deeper dissatisfaction with the oppressive structures of society all around him. T is rebelling against society in general, and Mr. Thomas' house just happens to be an unlucky and convenient target.
he does not hate Mr. Thomas but hates the beauty around him he cannot let anyone live peacefully because of the frustration in his mind left because of bombings.