Did you activate the quest to wipe out Nelson by Forlorn Hope?
Nope. When I went to Nelson, I killed everyone over at Hope. The thing I'm wondering is if they turned hostile because when I first got to Nelson; I was wondering if they would turn hostile if I killed the crucified NCR soldiers. I did kill them by beating them with a machete, but they never turned hostile. So, after that, I talked to Dead Sea, completed the quest to take over Hope, and after that everything was fine. I just happened to fast travel back to Nelson once, a long time after doing that quest and every Legion member there was hostile. I mean, I don't mind crouching and using a Stealth Boy to make them non-hostile, but I don't have an unlimited amount of them. Now I have to unfortunately avoid Nelson all the time now.