Jack Ruby was the most loyal, ass-kissing, mob wannabe that ever existed. He was willing to do anything to gain favor in the eyes of his bosses. There is one (1) and only one reason Ruby kills Oswald - because a mob higher up told him to.
There was some horse manure nonsense put out about him being so concerned about Jacki Kennedy. What utter crap. This is a guy who did nothing his whole life but exploit women. When one of his strippers turned up pregnant, he took her for an abortion, then made her work that night, and deducted the cost of the abortion from her pay. Yea, he had great concern about women's feelings.
The only way he kills Oswald is if he is told to by his higher ups. And the only way his higher ups waste a perfectly obedient ass-kisser like Ruby is if there is something Oswald could tell the police that they REALLY don't want out.
Wasn’t Ruby terminally ill at the time, too?
Jack Ruby grew up as Jack Rubenstein in Chicago. One of his boyhood friends was Johnny Roselie. Roselli was the Don of Los Angeles. When Oswald fled from the Texas Book Depository he ran to a movie house. The plan for Dallas PD was to take out Oswald, but he ran. In the movie theatre when Oswald saw the cops running in with guns drawn, he stood up raised his hands and gave up. The cops couldnt shoot him down in front of witnesses. Plan was Oswald had to go; dead men dont talk. Roselli contacts Ruby and Oswald dies. All planned. Oswald told the truth when he told the reporter he didnt kill anyone, he was a patsy and was set up.
They almost whacked JFK in Tampa several weeks before but the hit was called off on scene minutes before the shooting started. Giancana (Chicago) Trafficante (Tampa) and Marcellos(New Orleans) all wanted JFK dead.
Not a good bunch to have after you. Not at all.