Why does the lynch mob go after T.J.?

TJ went to the Barnett's store with the Simms brothers. The boys tell TJ its alright to take the gun he's been wanting and pay the next day. TJ doesn't seem to understand the boys are setting him up, but while they're in the store, Mr. Barnett comes down to investigate and struggles with Melvin, until RW hits him over the head with the flat side of the axe. Later, the Simms boys say they saw TJ running away from the crime with some other boys. The lynch mob is there to punish TJ for his actions.