It was hard to hear everything splayed out in the open. He wanted to cringe away when James leaned forward, but he knew that there was nowhere for him to go. They were pressed tightly up against the walls of the tunnel, and short of actually leaving, there was no way that he could avoid James.
"I am them," Sirius half-whispered when James said he was afraid of being like them. And that was the truth of the matter. He had been foolish at sixteen to think that he could run from them. He couldn't. Because the darkest part of them was always inside him. And he could never escape that, no matter what he did. There was something implicitly rotted about him; there were plenty of people who could have walked into that library, read the same things he had seen, gained the same knowledge he had, and not made the same decisions. But he had a natural inclination toward it, a propensity that was foolish to ignore.
And that was the scariest part of all of it -- and that was what Rodolphus claimed time and time again, and it was always most difficult when he was speaking the truth: They had merely shown him what he had always the potential to be. There'd been little forcing on that spectrum. He'd walked through that door on his own, and Rodolphus was right; he never knew what was inside his head now, and what could come out to hurt James and Lily again.
He was never happier than when he was pretending things were normal with them. The night of drinking with James and Remus -- he'd finally felt sane again, comfortable in his own skin. But it was because he was pretending that those years hadn't happened, that he could just strike them out. And he couldn't do that, no matter how much he wanted to.