Wow. Unless Tony's 'present' point in time was before the events of Siberia, that must have taken quite a bit of restraint. To Tony's credit, if he had made a scene - around Bucky or otherwise - Steve hadn't noticed it. He hadn't even been aware that the two had interacted until just now. He'd have to thank Tony in some form or fashion later. Because that was no minor feat.
Steve thought about dancing around the subject again. It felt like it was too much on top of an already existing mountain of awful. At the same time, Natasha had made a really valid point. If Bucky didn't find out through Steve, he'd find out through someone else. Likely Tony, in this particular scenario. And that reveal would be a lot less pleasant coming from Stark.
Steve took Bucky around the corner and down a hall. Away from the lobby, out of anyone's immediate vision. He'd finally slow them to a stop, turning to face his friend. "You were on a mission. I can't stress how much this isn't your fault. You were being controlled, you were just doing what HYDRA had told you to do. But.. you killed Howard and his wife. And when Tony found out about it.. sort of put a divide between us."
Well. Technically they were already having something of a quarrel because of the Sokovia Accords. But Siberia had really caused things to get out of hand. Steve leaned away, giving Bucky some space. "Do you.. want to go back upstairs?"