Here’s a quote from “When the News Went Live,” page158, by Bob Huffaker:
“But early in taking depositions, Griffin distrusted the police, and he doubted that Ruby had admitted deciding to kill Oswald when he saw the assassin at the midnight showup. Ruby had bragged that he’d known he was going for Oswald when he’d seen “that smirk on his face” before the cameras, and Pat might have misunderstood which Oswald photo opportunity he’d meant. Belli had argued that Ruby, having been armed at the midnight showup, could have shot Oswald then, but Ruby would later confess to Earl Warren that he’d lied about carrying his pistol that night. The Sunday transfer, then, would have been his first chance to get a shot at Oswald.”
So the “smirk” appears to have been a contributing factor for Ruby’s actions.