What are the gang's motivations for destroying the house in "The Destructors"?
The plan to destroy Old Misery's house originates with Trevor ("T"), who supplants Blackie as the group's leader with his audacious proposal. T's motivation stems from his anger at how his family has fallen in status after his father has lost his position as an architect. T recognizes that Old Misery's house was designed by noted architect Sir Christopher Wren, and it is a way for him to symbolically strike back at a world that has rejected his father. T's motivation is also his desire for acceptance into the gang, and wresting leadership away from Blackie is an additional benefit.
The other boys' motivations for destroying the house are much more mindless. The destruction that came with The Blitz has desensitized them; they see the ruins of other houses littering their landscape. It doesn't occur to them that there is any real additional harm in their actions as they begin to tear the house apart. Although they don't have any fondness for Old Misery, neither do they have any particular hostility towards him.
In "The Destructors," the gang's motivation for destroying Old Misery's house is three-fold. First, the gang wants to raze the house to improve their prestige among the other street gangs. Blackie and the others feel that such a stunt would bring the Wormsley Common gang recognition and respect.
Second, the gang wants to destroy Old Misery's house because they distinctly do not trust Mr. Thomas. Even after his attempts to ingratiate himself by giving the boys some Smarties candy, the gang members still eye him with suspicion, commenting that he probably found the candy or stole it.
The final motivating factor for destroying the grand old house is to rebel against society. The boys collectively detest what the opulent old home formerly stood for, which of course is the superiority of the upper class. By tearing down the old house, the boys effectively declare their own independence and strength against the hierarchy of society.