Mr. Thomas's house symbolizes the hope of a better world amid the ruins of post-war London. But the members of the Wormsley Common gang have been too brutalized by their experiences of growing up in poverty and neglect to realize this. To them, there's never been any hope. They are children, but they are far from innocent; they've been forced to grow up too quickly by their harsh environment and upbringing.
Trevor's situation is different. Coming from a middle-class background, with all the trappings of money and social respectability, he really should know better. Like his fellow gang members he has no innocence, but the difference is that he's lost his, whereas the other boys never had any to begin with. Trevor's sudden fall in social status has brutally robbed him of a life of ease and comfort. Having lost all hope and innocence, T—as he now prefers to be called—is filled with nihilistic rage and despair. He sees no hope for the future. His lack of self-respect is mirrored by his lack of respect for other people and their property. And so he eggs on his new friends to reduce Mr. Thomas's house to rubble.