T.J. Avery is one of life's troublemakers. Lazy, ignorant, and proud of it, he's always getting himself into mischief of one kind or another. But even T.J. excels himself when he participates in the robbery of Barnett Mercantile with the Simms brothers.
The robbery ends with the death of Mr. Barnett, hit over the head in the course of the hold-up by one of the Simms brothers. Although T.J. wasn't directly responsible for Mr. Barnett's death, his involvement in the robbery will land him in very serious trouble indeed.
So why did T.J. do such a terrible thing? As we've already seen, he's a serial troublemaker, and so there's a horrible sense of inevitability about his participation in an armed robbery. Also, T.J. had no time whatsoever for Mr. Barnett, a thoroughly unpleasant racist and white supremacist who seems to revel in humiliating and degrading Black people when they come to his store. In an earlier interaction with T.J., Mr. Barnett blatantly ignored the young man to serve white customers.
One can also attribute T.J.'s participation in the robbery to a deep-seated desire for a sense of belonging, a need to be part of something bigger than himself. Rejected by virtually everyone in the local community, T.J. most probably feels that being part of a criminal gang will give him the sense of solidarity that he needs and which his current life patently lacks.