The truth is that Ruby really was insane. That wasn't a lie. You only have to read his testimony to the Warren Commissioners to realize that he was out of his mind. And because of that, nobody was going to trust him to pull off a killing. Nobody was going to feel in control by putting a gun in his hand.
Some people foolishly claim that Ruby was prodded by his Mob/CIA bosses to kill Oswald, but they don't get it. The Mob and the CIA were never going to trust Oswald with killing Kennedy, and not because he was insane, but because he wasn't an assassin and he wasn't even a marksman. The Marine record shows that Oswald got one point above failing on his final shooting test in the Marines. That was in 1959, and after that, the only shooting he did was that when he was Russia, he went rabbit hunting a few times with his Russian pals. But, they used shotguns, not sniper rifles. And they said that Oswald still couldn't hit the broad side of a barn. Seriously, they said that he never hit anything. So, why would the Mob or the CIA choose Oswald to shoot JFK? He'd have been just as likely to hit Jackie as JFK.
And it was the same with Ruby, except that in addition to that, he was insane, as in stark raving mad (which Oswald wasn't). Do you think anybody wanted to put a gun in Ruby's hand? Of course not.
Jack Ruby was just the face of the Oswald assassination. He had the exact same role in it that Oswald had in the JFK assassination; as the patsy. They killed Oswald themselves, just as they killed Kennedy themselves. They weren't going to trust either one of those patsies to do any killing.