Ruby didn't die until 1967. Did he know he was terminal in 1963?
His first trial ended March 14, 1964. He was issued a ticket to the chair, despite Melvin Belli's best efforts.
His first lawyer, local attorney Tom Howard, had asked him if there was anything other than shooting Oswald on national television that might damage his defense. Ruby brought up only his involvement in a gun running scheme in the service of anti-Castro forces.
Ruby swore to the Warren Commission that he was not part of a conspiracy. But a year later, in 1965, he changed his story and began hinting about a conspiracy to overthrow the government and claimed that he knew who had Kennedy killed.
More than a year after that, on October 5, 1966, Ruby's lawyers succeeded in getting his murder conviction overturned, on the ground that the original court should have granted a motion for a change of venue. A new trial was ordered, to be held away from Dallas, in February 1967.
But Ruby fell ill December 9, 1966 and died three weeks later from cancer of the liver, lungs, and brain.
So, it seems likely Ruby was telling the truth when he denied to the Warren Commission that he was part of a conspiracy. His later deviation from that line was probably an effort to postpone his date with Old Sparky for as long as possible. His likely motive for the killing: he was royally pissed off that a pro-Castro twerp had murdered a president he admired, and he was just nuts enough to act on his anger.
Yes, Ruby knew he was terminal, I lived through those times, the fact he had cancer was known and reported on the news almost immediately, and he died 4 years after killing Oswald, 4 years is a short time. Jack Ruby was a known criminal, he was not exactly your patriot type. He had other reasons(probably hired to do so)for killing Oswald, it certainly wasn’t out of love for country or Kennedy.