Sarcasm was one way of coping, Harry had discovered. The time he'd inadvertently gotten them into trouble with his journal and talked to Death Eaters, he'd used it to cover his fear. Now - all it hid was exhaustion. Not physical or mental, at least not at the moment. He was just tired of conflict, but it seemed there was no end to it.
Which was probably why he said more than he should have, but he just wanted this confrontation to be over. "Yes, I did," he said. "I didn't manage everything. There were a lot of people I didn't, couldn't save. I'm sorry that one of them was your friend. But it wasn't because I did nothing, because I didn't try."
That rankled, a little bit. He understood the blame for deaths; he felt the weight of those on his conscience all the time. But Smith seemed to think he'd just hidden for five years until someone else had killed Voldemort. That was not what had happened. Harry didn't feel the need for credit for his actions, he didn't need to be received as a hero; that would have made him uncomfortable. He didn't appreciate being blamed for something that he hadn't done, though.