There was not one episode in general that showed Tom never harmed anyone. The one you are referring to is probably when Atticus showed how Mayella was beaten about the face and neck on the right side. Then he showed Tom's withered left arm caused by an accident when he was a child. A person with this disability would leave bruising on the side opposite their dominate hand. Tom could not have beaten Mayella, but her daddy was left handed. He could, and probably did beat her when she tried to seduce Tom.
In addition, Tom was a hard and respectful worker, as shown by his white employer's testimony. He also helped Mayella whenever she asked him to. Tom felt sorry for her and the lack of help she had from the Ewell family.