Why do they destroy Old Misery's house in "The Destructors" by Graham Greene?

They destroyed Old Misery’s house because it was there, and because mob mentality took over. 

The boys in this story destroy the house for fun.  It is a way to pass the time.  It sounds very odd, but it was one of those situations where one person suggested the idea, they all went along with it, and they could not seem to stop.  

Old Misery is a bit eccentric, hence the name.  His house...

They destroyed Old Misery’s house because it was there, and because mob mentality took over. 

The boys in this story destroy the house for fun.  It is a way to pass the time.  It sounds very odd, but it was one of those situations where one person suggested the idea, they all went along with it, and they could not seem to stop.  

Old Misery is a bit eccentric, hence the name.  His house was not in good shape to begin with.  After the bombings, he tried to rebuild his house as best he could.  However, he had a reputation as a cheapskate. 

[It] was common knowledge that since the bombs fell something had gone wrong with the pipes of the house and Old Misery was too mean to spend money on the property. He could do the redecorating himself at cost price, but he had never learned plumbing. 

One of the main reasons the gang became destructors is because of the new member, T.  The gang should have resented T, but instead they were enamored of him.  He told them that he had been inside Old Misery’s house, and it was beautiful.  It is T’s idea to destroy the house. 

Blackie said, “Nobody’s going to pinch things. Breaking in—that’s good enough, isn’t it? We don’t want any court stuff.”

“I don’t want to pinch anything,” T. said. “I’ve got a better idea.”

“What is it?”

T. raised his eyes, as gray and disturbed as the drab August day. “We’ll pull it down,” he said. “We’ll destroy it.” 

In order to maintain his status in the group, T has to suggest something serious and explain why he went in the house.  The boys are bored, and jaded.  They amuse themselves by destroying the house because they are there, and it is there.  Any boy who didn’t would be seen as a coward.  That is how the mob mentality works. 

T tells the boys he doesn’t hate Old Misery.  It would be no fun if he did. He seems to be a sociopath.  However, he has a charisma that leads the other boys to follow him.  Therefore, one boy turns a group of boys into destructors.