She was trying to protect him by not making him riminding past and to make him living a normal life. You're right about the killing Sandy part, but Better that to do it Is a person who is already fucked. Plus she admitted having used him, but She cares about him and still wants to protect him. Last thing, I don't think Grace is really dead. Jacky, even if he's extremely suggestible, he still has a bit of lucidity to not to kill Grace, who's one of the few people (together with Paige, Candy, his son/daughter and even Kenzie I think) he really cares about. I think he loaded the gun with the blanks before shooting her