Firstly, note the way that T. disposes of the savings of Old Misery that he finds stashed in the mattress. It is clear from what T. does with the money that he has not destroyed Old Misery's house for purposes of gain or personal enrichment. Note what he says to Blackie:
"We aren't thieves," T. said. "Nobody's going to steal anything from this house. I kept these for you and me--a celebration... We'll burn them," he said, "one by one."
The way in which they burn the notes seem to symbolise a chilling sense of nihilism through rejecting what the world places value in.
Likewise, it is clear that when the boys lock Old Misery in his own outside bathroom that personal vengeance and cruelty have nothing to do with their act of destruction. Note what is whispered to Mr. Thomas when he is safely locked up:
"Don't worry, Mr. Thomas," it said, "we won't hurt you, not if you stay quiet."
This impression is further reinforced by the way in which they give him food and a blanket to keep himself warm. They obviously do not want him to be hurt or damaged in anyway, at least physically.
One motive that you can eliminate is theft. When T. speaks to Mr. Thomas, T. rings the doorbell instead of breaking into Mr. Thomas's house, and T. doesn't pinch, or steal, anything. Instead, T. simply wants to see the house, and he refers to it as "beautiful."
While they are destroying "Old Misery's" house, the boys find the man's savings in his mattress. T.'s coat is stuffed with bills, but he decides not to steal the money. Instead, he says, “We aren’t thieves...Nobody’s going to steal anything from this house." T. decides to have what he calls a "celebration." He puts the bills on the floor and counts them. Then, the boys take turns lifting each bill and holding it by the corner while they burn it. T. also says that he doesn't hate Mr. Thomas. Instead, his goal is destruction--pure and simple--not theft or any act of hate.