Why would it be wrong to corrupt Tom Robinson's innocence?

In this context, the term corrupt can refer to a dishonest attempt to make Tom Robinson appear less innocent than he is or to debase his innocence by altering or misrepresenting his testimony or character. Such acts, by their very nature, are wrong. 

The solicitor Mr. Gilmore sets out to do precisely to Tom what is mentioned above when, in chapter 19, he cross-examines him. He first recalls Tom's earlier punishment for an altercation that...

In this context, the term corrupt can refer to a dishonest attempt to make Tom Robinson appear less innocent than he is or to debase his innocence by altering or misrepresenting his testimony or character. Such acts, by their very nature, are wrong. 

The solicitor Mr. Gilmore sets out to do precisely to Tom what is mentioned above when, in chapter 19, he cross-examines him. He first recalls Tom's earlier punishment for an altercation that put him in jail and then questions Tom's motives when he helped Mayella Ewell each time she asked him for a favor. When Tom states that he felt sorry for Mayella, Mr. Gilmore makes a big thing out of it since he knows that many of Maycomb's whites will feel offended that a black person can dare to feel sorry for one of their own. They will be outraged at the mere thought.   

Moreover, Mr.  Gilmore intends to impress upon the court that Tom ran away at Mr. Ewell's arrival because he was guilty of sexually assaulting Mayella. Even though Tom tries to make it clear that he fled because he feared retribution for something he had not done, the mere act of escape is seen as a damning indictment against him.     

It should be pertinently obvious that Tom is a hard-working and respectful man. His only purpose is to help those he believes are in need. On the day of his supposed crime, he felt compassion for Mayella Ewell because he realized how broken and destitute she truly was. Of all the citizens in Maycomb, he was probably the only one who, at the time, recognized Mayella's need for comfort and attention. His intentions, in that instance, were pure, and in any situation, such care and compassion should be praised rather than condemned.    

Corrupting Tom's innocence by stating that he is a bad person with ulterior motives just because he is black is not only completely wrong but also grossly perverted, unjust, and deliberately malicious.