If I recall correctly, I think he said that he didn't want the widowed First Lady Jackie Kennedy to have to suffer through the pain and grief of Oswald's trial. Maybe he was lying, but it's hard to say. Oswald himself said that he didn't do it, and maybe he was lying about that, but he never got the opportunity to defend himself in court (although I think someone conceived a dramatization of a hypothetical trial of Lee Harvey Oswald).
I don't know how Jack Ruby might have really felt about JFK, although I do know that in Dallas, there were quite a few people who hated Kennedy. The right-wing was still pretty much against JFK and hated him vehemently. The KKK still had some strongholds in Texas, and JFK's stance on Civil Rights definitely did not win him many friends south of the Mason-Dixon Line. A lot of the militarists and hardline Cold Warriors were also against him, thinking he was too soft on communism (which was never really true, but that perception still persisted with a lot of people). J. Edgar Hoover absolutely detested the Kennedys - and Martin Luther King, for that matter.
So, with all these people on the right-wing seething with hatred for JFK, it does seem rather uncharacteristic that the purported assassin was a left-wing Marxist-Leninist. I don't think the Soviets or any of their allies would have benefited from Kennedy's death. Oswald is generally portrayed as a drifter and a failure - a bum who was probably mentally disturbed. The Soviets obviously had no use for him. He was a bum. But his name will be in the annals of history for generations to come.