Her gaze remained steady on his as he answered his own question. And then it came time to have the conversation she'd come there to have. She didn't want to, didn't want to admit to him of the things she'd done. But he had a right to know. The last thing she needed was Amy telling the Doctor about his death at River's hands. Remaining where she was, her gaze followed him as he started pacing. "On Demon's Run after I was born, I was taken by a woman called Madame Kovarian. Her scientists experimented on me and turned me into a weapon to use against you. To one day kill you." She watched him carefully, wanting to see his every reaction. By telling him all this, she was breaking almost every rule of time travel, but him finding out this way was for the best, wasn't it? "In 1938 Berlin, that's what I did. I tried to kill you, but you out-smarted my every move for a gun. But the one move you didn't protect yourself against was my kiss. I used a poison with no antidote. Eventually, your pleas for River Song, someone I had no idea who you were talking about because that wasn't my name yet, and your devotion to my parents got through the good in me. I saved your life, but at the cost of using the rest of my regenerations. I can't regenerate anymore, but I don't regret it. I'd always give them all up to save you." Which was the truth, she would always give up her lives for him.
And in a way, she died for him twice. First there'd been the sacrifice of her regenerations. Then there was the time in the Library. So many times the Doctor saved others, but rarely was he ever saved by someone. Of course there'd been other companions that had saved his life, but had any of them made larger sacrifices than River? It was hard to say. But River truly hoped that her two acts of saving the Doctor made up for the crimes she'd committed. And she now waited for the Doctor to lash out at her or tell her she was stupid for saving him and giving up so much. But in truth, the universe needed the Doctor. It could get by without her, but not him.