Ruby spun around and found Dean about to stab her with her knife. That was just so wrong. She grabbed it and they began fighting. Sam ran in and pulled Dean off of her.
Then he was telling her to leave. She went out the door but stayed to eavesdrop. This was the moment she was waiting for. Sam would choose between her and Dean. She honestly wasn't sure which way it would go. On the one hand, they had this quarter century long co-dependent relationship. On the other hand, she was Sam's drug dealer and Dean wanted to cut him off.
When she heard the fight start she smiled and went to get the car. She knew Sam would be down shortly.