From what Bucky had observed over his time in Starklandia, no one was coming from the exact same moment. That meant there were always stories to be told, information that needed to be exchanged whenever someone new arrived in town. Bucky had been quietly reading exchanges between others, gathering information when he felt he needed it. But no one thus far had come from as far ahead as Natasha. No one really knew the full aftermath of what Thanos had done and how the world was coping with it until she'd arrived. All Bucky knew was that he was one of the ones removed from existence when Thanos snapped his fingers. He'd been angry when he found out, but even that news paled in comparison to what Natasha began to unfold for him.
It took some time, understandably, for the remaining Avengers to try to figure out what to do to 'fix' things and bring back those they'd lost. As he listened, it took a decent amount of self control for Bucky not to turn his head toward one of her hands and brush his lips to her palm. The urge was there, but he kept his composure while she spoke -- a first.
Of course Steve couldn't move on. That was Steve. He was going to hold onto the pain of what was lost for the rest of his life unless it was fixed. Hearing about the others wasn't surprising either, but it was still upsetting knowing they seemed to be stuck in an endless loop of guilt and not knowing what to do to keep moving forward. Hearing that she'd thrown herself into trying to keep the Avengers going wasn't surprising either. Natasha wasn't going to be able to stop. It had deeply bothered him, knowing what he knew of his own fate and that he'd not told her he remembered their time together before it was too late. He felt awful about that, but even that couldn't compare to how he felt after she continued her tale.
Hearing that the remaining Avengers had come up with a plan to get everyone back was really pretty humbling for Bucky. Of course they'd want to restore the universe, and those who were unjustly taken away. But knowing there were people out there who were working tirelessly toward finding seemingly impossible solutions because they wanted to bring them back was pretty amazing, really. What wasn't amazing? Finding out the cost someone had to pay. Namely, her.
It took Bucky a few seconds to really believe what she was saying, that she'd given her life on the chance that they'd succeed in bringing back everyone else. In other words, she'd killed herself and was hiding that fact behind the cloud of the greater good. Given that they were looking into her eyes, Natasha would see the realization come into his gaze, followed by the stormy flash of anger rising in him.
"You what?!" He said, his tone shifting from disbelief to accusatory. "What the hell were you thinking?" Bucky didn't let her go at first; he held her tighter. "How could you do something like that?! Your life is not invalidated just because he had a wife and kids. That's not worth you losing your life!" He did let go of her then and took a step back, rubbing his face with his human hand. "Jesus fucking Christ." Bucky had seen some dark shit go down but this really hit a nerve. "You want to tell me you did it for the greater good? Is that it? Because I'm not seeing that when it turns out with you dead. There's no 'best' scenario if you're not in it."