Ruby taking his dog with him = no intention of shooting Oswald.
So, all that leaves is a sudden impulse to do it when he got there. But, if that's the case, then why couldn't he remember it? He wasn't in a trance. He remembered what he said right before and right after. He said before, "You killed my President, you rat." And, he said after when he was swarmed by cops, "It's me, Jack Ruby. You know me. What are you doing?" So, if he could remember those things, how could he not remember the shooting? Jack Ruby had no memory, no mental vision, and no awareness of shooting Oswald. When asked about it, he said it was a "blur." But, how could it be a blur if he remembered what he said right before and right after?
The reason he had no memory of it is because he didn't do it, and it didn't happen. He said what he said to Oswald, and then he was swarmed by cops. And then he said what he said to them. Period. There was nothing else. There was nothing in-between. He did not shoot Oswald.