What he knew about Natasha was limited mostly to what she or Steve had told him. It was easy to see she'd been at this a long time, but apparently he had no idea of just how long - and at just what cost her skills came.

Then there's the Winter Soldier's involvement. Steve had been adamant about how little of it was under Bucky's control, but he'd been cognizant enough to pick a sixteen year old girl to train. He doesn't know what's worse -- that he'll kill so many people for something he doesn't believe in, or that he'll teach someone else how to do the same.

"And what happened after that? She said you and I... did a little more than training."