This is an image from Murderpedia. They have a nice page on Jack Ruby. But, read the caption below the picture. Even they acknowledge that Ruby could have had no plan to kill Oswald since he brought his dog Sheba along.
What this proves is that the whole idea that someone, such as the Mafia, went up to Ruby and said, "You have to kill Oswald" is RIDICULOUS. And it's ridiculous for multiple reasons. It's ridiculous because he brought his dog along. If he knew he was going to be killing Oswald for the Mafia, he would have made arrangements for his dogs- knowing that he wasn't coming back. But, it's also ridiculous because killing Oswald, with his capture a certainty, was so ruinous and destructive to his life, it was like the end of his life, and it is inconceivable that the Mafia could have threatened him with anything to sway him to do it. "Kill Oswald or else we'll..." What? Fill in the blank. What could they have threatened? I mean, he didn't even have a family, a wife and kids, that they could threaten to kill. So, what, did they threaten to kill his sister? It is ridiculous. That dog in the car proves that Ruby had every expectation of driving home that morning after not having shot anyone. Furthermore, if he actually planned to kill Oswald, don't you think he would have gotten there on time? The transfer was scheduled for 10:00.
So, all Ruby accusers are left with is the idea that the sight of Oswald triggered a sudden impulse in Ruby to kill him. But wait. Ruby had caught sight of Oswald before, in the hallway, and at the Midnight Press Conference, and presumably he was armed at the time. If the sight of Oswald was going to drive him crazy, then why didn't it? Nobody reported him to be the least bit agitated at those sightings.
Ruby walked down that Main Street ramp, just as he said. Nobody opened any doors for him. He cited a cop who was there at the time: Rio "Sam" Pierce. How could he know that Pierce was there unless he was there himself? That is definitely how Ruby entered the garage. He was lured to the Western Union office, which was down the street, by Karen Carlin, one of his strippers, and I suspect there is more to her story than we know. The big question is how he was nudged to go down there. If he knew the jail transfer was scheduled for 10, wouldn't he assume that it happened at 10?