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 conflict with society and all that it represents. But this is only partial resolution. T still hates society with a passion, and one senses there are many more such mindless acts of destruction to come.
Identifying the protagonist and antagonist in Graham Greene’s short story “The Destructors” requires a departure from the conventional wisdom regarding definitions for both characters. In “The Destructors,” the protagonist is Trevor, or "T." as he is called by the other boys in the Wormsley Common gang. T. is anything but heroic. He is the protagonist, however, because he is the central figure in the story and the one who provides the motivation for the action that consumes most of the narrative: the destruction of the Christopher Wren-designed house owned by Mr. Thomas, or Old Misery as the boys call him. Not only is T. unheroic, he is most likely a psychopath. As Greene’s unseen narrator states regarding T.’s ambitious plan to systematically destroy the house and the gang’s success in so doing, “A kind of imagination had seen this house as it had now become.” T.’s psychosis is also evident in the narrator’s description of the new gang leader’s reaction to the news that Mr. Thomas is returning early from holiday:
“Old Misery,” Mike said. “He’s on his way.” He put his head between his knees and retched. “Ran all the way,” he said with pride.
“But why?” T. said. “He told me. . . ” He protested with the fury of the child he had never been, “It isn’t fair.”
If T. is the protagonist, then the antagonist is Old Misery. Mr. Thomas is the antagonist solely because he would, if he could, defy T.’s actions and do everything in his power to stop them. The way Greene presents his story, one expects Mr. Thomas to be a harsh, unfriendly tyrant. There is nothing in this story, however, to suggest that he is anything other than an aging pensioner simply going about his business. In fact, when T. visits Mr. Thomas’s house and asks to view the interior, the old man graciously invites him in, unaware that the visit is a ruse to surveil the abode in preparation for the plan to destroy it.
The conflicts in “The Destructors” are several. Most importantly, of course, is the conflict between the gang and Mr. Thomas and his house. This conflict is central to the story. The boys hold no serious animus toward the old man. They merely wish to enhance their reputation by performing grand acts of little consequence. T.’s idea to destroy the house represents a major departure from the suggestions previously offered by the gang’s previous leader, Blackie. The reference to Blackie leads to the second conflict, that between him and T. for leadership of the gang. Blackie is the victim of a rather discreet coup d’état, dethroned by T.’s reputation for silently brooding and, more importantly, for coming up with the audacious plan regarding the house.
Another conflict in Greene’s story involves the Wormsley Common gang’s war with civilization. Like most gangs, the boys in this one seek to rebel against conventions. They are very young and limited in their capacity for felonious behavior (that is, until T. enters the picture) but aim to make their mark as outlaws any way they can.
In Graham Greene's post World War II tale, Trevor is the protagonist, who represents the nihilistic attitudes and behavior prevalent in the period after the destruction of London. His conflict develops from his rejection of everything that pre-War London, represented by Old Misery, valued.
The son of an architect, now only a clerk, Trevor has moved into a neighborhood quite below the social status to which his parents are accustomed. Since the boys with whom he becomes acquainted are not of his ilk, he vies for the top position in the gang by devising a plan to destroy the beautiful, albeit "crippled" home of Old Misery, a former builder himself. This act of destruction committed while Misery, who is on a three-day bank holiday, is not without a creative process as T, (whose aristocratic name has been cut) has determined that the gang will take it apart from the inside "like worms." While he orders the destruction of the interior and all its contents, T. yet retains some ethics: He does not want any theft; in fact, he orders Old Misery's banknotes burned and, later, food taken to Mr. Thomas, who has been locked in his outdoor lavatory [loo]. Nevertheless, the boys are devoid of any sympathy or other genuine feelings about Mr. Thomas's loss of home. As a result, they enter into conflict with many of the pre-war, old world moral issues represented by Old Misery; that is, those that forbid delinquency, trespass, and destruction.