Remember, the climax of the story is the key scene in the tale - that tense or exciting or terrifying moment when our emotional involvement is greatest. Now we learn what the outcome of the conflict is going to be. To my mind, the climax of "The Destructors", therefore, is when the boys have already advanced greatly on their mission of completely destroying "Old Misery"'s house, but the surprise return of Old Misery threatens the entire project and also T.'s leadership of the gang. Note how the gang is just ready to drop and leave everything and run, but T. insists that they continue:

"Anybody could do this - ". "This" was the shattered hollowed house with nothing left but the walls. Yet walls could be preserved. Facades were valuable. They could build inside again more beautifully than before. This could again be a home.

We see here T.'s obsession with the absolute destruction of the house and his fear that his project might be thwarted. Note too how T. reacts as it appears his plans will come to naught:

T. stood with his back to the rubble like a boxer knocked groggy against the ropes. He had no words as his dreams shook and slid.

Note the simile that compares him to a boxer knocked for six and defeated in the ring. It is only the support of Blackie that brings him round. When they lock Mr. Thomas in his outside toilet it is clear that they have passed the crucial danger - the final destruction can proceed in the denouement of the story.