There's a semblance of a smile, there, because Resistance people would like him. They'd see what he'd done as noble, somehow, as if all the blood he'd spilt while smiling meant nothing in the end.

Maybe it didn't, but Cassian's guilt tells him it did, and the knowledge that he would have removed himself from the entire equation too early to be any help to Jyn is a different kind of guilt, but guilt all the same.

"That would be something to see." Because Jyn doesn't really like getting along with anyone. She pushes, full expecting to get any real offer of friendship or family rejected outright or stolen from her at a moment's notice.