I think she saved him, when she was dying he was already forgetting her if she told him to go back with Sora he would think this girl is NOT to be trusted and that killing her was a good idea when if he remembered her fully he would understand better, and Roxas would most likely get turned into a dusk or or some sort of mind controlled zombie. Xemnas probably would pull a stunt like that, he does need a keyblade for kingdom hearts. Xion was saving Roxas from her fate as puppet. She might have not known that Riku was gonna beat him up but she did know that him freeing kingdom hearts will keep him away from the organization and at least turn on them for a good amount of time to him to find out where he really belongs on his own like she did, with Sora.
