By choice or by conditioning?
Unless Ra's has been given mind control powers, choice. They haven't established mind control powers in the Arrow universe, and if they do I doubt it will be with Sarah. As for "conditioning", that is such a vague term I could see it being used for someone on arrow, but probably not Sarah. Its as unrealistic as telepathy, but so vague it basically covers most mind control not psychic based. Anything from chemicals to putting a dog collar on someone and shocking them when they don't do what you want can be "conditioning". I could maybe see it because of that, but Sarah has been pretty clear that she holds herself responsible for the killing, and I don't see that kind of control just going aay one day, not if it had been effective for so long. Plus, storywise its a cop out for Sarah, and would kind of screw up her character if she hadn't been in control of herself.