There are many theories about why Jack Ruby shot Lee Harvey Oswald. Some believe that Ruby was telling the truth when he said that he did it to "spare Mrs. Kennedy the pain of a trial."
Others believe that he was sent as an assassin, himself, to shut Oswald up before the truth of the Kennedy assassination conspiracy could come out.
Either way, I don't remember ever hearing anyone say that he shouldn't have done it, in the years following the act. Whatever his motives, Ruby channelled the wrath of the American people against the man they blamed for the end of an era. At least, that's how it seemed to me at 5 or 6 years of age, listening to the grownups around me still talking about it 3 or 4 years after it happened.