He wonders if he made a mistake. The truth will have its day no matter what Steve does today. He's learned that much from this experience. Secrets caused all this destruction to begin with. That doesn't make Steve wonder any less if he isn't signing Tony's death warrant. He doesn't respond to his accusation because no he really wasn't. It was too hard; too personal to take to someone he barely knew. It does mean something that Tony is still talking about helping him after this. It's enough to get him to respond even though his gaze is far away. He's numb now to everything.
He continues in a matter of fact manner, "The man I knew was ruthless enough to take on a monster who tortured, mutilated, and killed soldiers from across the world if he thought he'd gain something from it. I don't want that to be my friend, but it's the truth. I told him everything. I gave him everything that I could recover. He knew what he was." The betrayal is plain to see on his face. Howard found a way to break his heart one last time. This is far worse than his involvement with the atomic bomb. He can totally see how Howard couldn't have known they'd actually use it much less against who. The rest is unforgivable. He truly hates him for allowing evil to flourish. And Peggy... well she certainly suffered for allowing this too, didn't she?
"But I can't believe that Howard would let one of us be used that way. Zola made a lot of implications. I can't be sure how much of it was to hurt me or is the honest truth. They were about your father so... it's up to you whether or not you want to hear them. All I can say it while we're here I have no way to confirm it one way or another." Because fuck it. Why not tell him everything he heard? It can't get any worse than this.