Definitely Wanda's attachment to the humans. I don't think she would give up that secret for any other species in the universe, nor do I think she would even be put in the position to make the choice she did on any other planet. This is the planet where she found love, found where she belonged, she never had that attachment before.
The humans, despite their good intentions would continue to murder the souls in their pursuit to extricate them from the body, what was Wanda to do? She could never turn them in, her loyalties were no longer with the souls even if she still loved them as her family.I think her regret for what the souls had done was pretty exclusive to what they had done to the human race...not necessarily to the other species.
Edward talks to me in my head.....they tried to give me medication for that but I wouldn't let them.