He knew that Hydra killed Stark’s parents, and he knew that Bucky was Hydra’s top assassin. He couldn’t be sure that it was Bucky himself who did it, but if he’s being honest (and he’s Captain America; he’s being honest) he knows that it probably was.
He’s mad because he’s still a spoiled rich kid. Cap didn’t kill his parents, didn’t know anything specific to tell Stark, and there’s probably a lot more things Stark knows that he’s not telling Cap. He’s just being a brat as usual.