This is an interesting question. In a story that is famed for its dysfunctional, nihilistic teenagers, their good qualities are not really examined! Still, I guess you can think about this question in terms of the motivation that the gang of boys show for committing their act of destruction. What is stressed throughout is that they are not doing this out of any malicious desire to hurt. Note, for example, the way they treat "Old Misery" when they have him trapped in the outside toilet:

"Here's a blanket," the voice said, and a long grey sausage was worked through the hole and fell in swathes over Mr. Thomas's head.

"There's nothing personal," the voice said. "We want you to be comfortable tonight."

The fact that the boys take care of Mr. Thomas's material needs - giving him a blanket and then some food - show their good qualities in not wanting him to be hurt.

You might also like to think about the way that this disparate group of young adolescents organise themselves incredibly well when they are united around the common purpose of destroying the house. As Blackie goes to the boys on the first day, for example, he arrives a little late, but sees how effectively they are working together:

They opened the back door to him and he came in.He had at once the impression of organisation, very different from the old happy-go-lucky ways under his leadership. For a while he wandered up and down stairs looking for T. Nobody addressed him: He had a sense of great urgency, and already he could begin to see the plan.

The fact that these boys are able to unite themselves in such a way can be viewed as a "good" quality, even if it is for the purpose of achieving such a "bad" outcome.

The boys, then can be considered to have good qualities in the way they care for Mr. Thomas and in their extremely effective teamwork. However, of course, it is their "bad" qualities that dominate the story and linger in the mind long after we finish reading.