We don't know who it was that was firing the shots. Personally, I think it was Frank Lapidus and that Keamy guy, seeing as how they went on a mission a while ago just by themselves, and Keamy was shown to be the guy that was shooting down the plates. It could also be the Others, but if it was the Other's I don't think it would have ended with Alex shouting "I'm Ben's daughter!", because while that makes sense, it doesn't make much literary sense.
Also, I don't think Rousseau is dead. She could be, but I honestly just think she's injured, like Naomi from the season before. Rousseau seems too big of a character to have just a quick death like that. Plus I personally believe she's lying about her backstory, so I'm still hoping we'll learn more about her.