He should point out that Nathan's death could be an accident -- unless the Hulk killed him on purpose, and they have no reason to rule that out yet. But Tony changes topics, and Bruce frowns, becoming concerned as he squeezes Tony's hand back.

"What are you talking about? When your company was making weapons?" He can see what Tony's driving at, he thinks, and he doesn't like where it's going because he's going to wind up agreeing with the idea that Tony has blood on his hands.

"You made things that could hurt people, and someone else took that power and destroyed things. You blame yourself for that. Tony, you're mixing your metaphors. Their blood is on your hands the way Nathan's blood is on my hands. The Hulk is directly responsible for his death; he killed him. And I killed him too, by proxy."