In Harper Lee's To Kill a Mockingbird, what on page 119 shows that it was right or wrong of Atticus Finch to defend Tom Robinson? Why or why not?

As page numbers vary per publication of Harper Lee's To Kill a Mockingbird, it's difficult to point to the specific passage you have in mind. However, in Chapter 9, fairly early on in the book, Atticus makes some comments about the case to his brother Jack that clearly show the unfairness of Tom Robinson's case and show Atticus is morally correct and obligated to defend Robinson.

While Jack is staying with Atticus for Christmas, Jack gets the opportunity to ask Atticus about the case; specifically, Jack asks how bad the trial will be. Atticus responds by explaining that no concrete evidence exists in the case to prove the crime actually even took place, let alone to prove that Robinson is guilty of the crime:

It couldn't be worse, Jack. The only thing we've got is a black man's word against the Ewells'. The evidence boils down to you-did--I-didn't. (Ch. 9)

In other words, all that's being used as evidence in the trial is the verbal testimonies of the plaintiff, who is Mayella Ewell, and other key witnesses, such as Sheriff Heck Tate and Mayella's father, Bob Ewell. In explaining the above to Jack, Atticus is actually exposing a critical problem with the case. US court of law acts based on the legal principle corpus delicti, which means in Latin "the body of [the] crime" (Wex, Legal Information Institute, Cornell University Law School). The principle asserts that there must be proof a crime actually took place "before an individual can be tried for the crime" (Wex). Mere "you-did--I-didn't" evidence does not serve as legal proof that a crime of rape actually took place. Since Atticus knows Robinson is being tried unfairly and illegally, Atticus knows he is morally obligated to put his all into defending Robinson. In addition, regardless of evidence, all defendants in a US court of law are entitled to a defense; therefore, Atticus knows he would not be upholding his responsibilities as a lawyer if he did not put his all into defending Robinson, which further proves that Atticus is right to defend Robinson.