I would argue that T is the protagonist in that he's the one responsible for the main action in the story; he's the one who comes up with the idea of destroying Old Misery's house. That being the case, Old Misery would be the antagonist. Or, to be more precise, it's the traditional society that he represents which so antagonizes T.
T's family have come down in the world, and so T deeply resents his lower social status. He's come to feel a deep-seated resentment against society and all it represents. Inciting the Wormsley Common Gang to a mindless act of destruction is his way of getting back at society in which he no longer feels he belongs. T has been in conflict with society ever since his family came down in the world. Bitter and twisted, he's been itching at the chance to take his revenge. Old Misery's house presents him with just such an opportunity.
And once that big old house has been reduced to a heap of smoldering ruins, T has finally achieved some resolution to his long-standing...
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