Steve used the few seconds it took Stark to respond to try to pull himself together. This entire exchange was quickly headed south - was probably already there, to be honest - and Steve needed to get his temper under control before he did something rash. Unfortunately, the look on Stark's face was every inch a dare, and Steve had never been good at backing down from a dare. Bucky'd hated that about him, had complained more than once about Steve's need to prove himself, although he'd always been there to pull Steve out of whatever trouble he'd gotten himself into.
The trouble that Steve was verging on getting himself into right now, however, probably wouldn't have an easy out.
Stark was too smart to believe what he was saying, and the sarcasm he'd injected into his words backed Steve up on that assumption. Stark must have known his parents had been assassinated, but he wasn't willing to say it out loud. Just like no one else in the Capitol was willing to point to the Games and call them what they were.
More than a challenge though, Stark's words were a threat. Steve could recognize that easily enough; one wrong word from him, and Stark would probably go running straight to Stane with a report, and then a week from now, maybe two, there'd be a horrible factory accident in District 8. Because Stark acted as if Steve hadn't had his own taste of punishment, of the Capitol's retribution. Erskine had supposedly died of an allergic reaction, after all, but Steve had known it for the message it was supposed to be. It was true that the Capitol held all the cards, that daring to speak out against them only got people you cared about killed. But what Stark didn't seem to realize was that even if Steve didn't drink himself stupid at Capitol parties and make an obnoxious spectacle of himself, he still generally toed the line. He showed up when he had to, and he usually kept his mouth shut.
He couldn't fault his fellow victors for not speaking out, not exactly. But he could fault them for putting weapons in the hands of Peacekeepers, for acting as if they were having the time of their lives in the Capitol, while the rest of Panem starved, for giving off the impression that they were some of Stane's closest supporters. For Stark to sit there and act like the only response to the Capitol's cruelty was to buddy up to the system - to become another cog in the machine, doing the Capitol's dirty work - was cowardly and selfish.
Abruptly, Steve rose from the table, glaring at Stark. "I think it's time for me to go," he forced out. He desperately wanted to call Stark out on his lies, the words were right there on the tip of his tongue, but he swallowed them down, keeping them at bay by thinking about what the wrong words would mean for his district, would mean for people like Clint, or Bucky's sisters.
It was a very near thing, but the worst Steve allowed himself as he headed for the door was a pointed, "Let's not do this again, agreed?" There was nothing to be gained from it, after all; it wasn't as if he and Stark were ever going to see eye to eye on anything.