It's so good that you mentioned him, because this is from the first article of the top of Google's search:

"In July 1974, however, he was seized by police in Mexico City and shipped back to Chicago. One year later he was bullet-riddled in his home in Oak Park, Ill., by unknown assailants. He had been scheduled to appear before the U.S. Senate Intelligence Committee to discuss his alleged involvement in a Central Intelligence Agency plot to assassinate Fidel Castro in the early 1960s."

Source: https://www.britannica.com/biography/Sam-Giancana

So, the guy was involved and worked with the CIA. Meaning, CIA could have been involved, too.

And they conveniently offed him to prevent him to talk. Such class.

Ruby's mob associations are probably exactly normal for someone in his line of work, but also they tell us they he wasn't committed to the rule of law. That's consistent with his statement about saving Jackie from a trial - he thought retribution was owed, not justice.