In the last 5 minutes of Get Out,
Rose is sprawled on the road, dying from the gunshot wound, and Chris is strangling her.
Why does she start smiling?
She felt that by choking her to death, he was proving her family's theory that black men are animalistic. Even as she's dying, she feels she's being proven correct.
She was smiling because she saw the police car and assumed Chris would get in trouble.
It seems to me that she smiled because she knew he couldn't do it. Rose knew Chris didn't have the strength to choke the girl he loved. He was never a fighter, and had told her earlier that
You're all I've got.
She is proven right, as a few moments later, he loosens up his grip. In fact, her smile might've reminded him of good moments together.
I don't agree with mike's answer, as there were no cop lights in the scene yet.
I'm starting to think that she was using one of her mother's hypnosis tricks. That is, she'd "trained" Chris to love her when she smiled at him. So she smiled when he was choking her, in order to trigger his involuntary reaction to treat her gently.