"Yeah," he said. "Ultron happened." He looks away from Tony for a second because he doesn't want to assign any guilt where it doesn't belong. This Tony hadn't built Ultron, of course. Steve doesn't want him to think he blames him for what happened. Hell, he wouldn't want the other one to think that either. They all had a part in Ultron. They can all take responsibility together - just, not the way that Ross thinks would be best.