Ben doesn't want to save every living person. He wants to save himself and his people. And they fall under the category of living. So if he doesn't save every living person then he is dead. So you kinda just asked why would Ben save his own life.

Also I was pretty sure he said every living thing on this island would be killed. I think he was just saying that to tell Jack how dangerous these people are and how strong they are. I don't think he is talking about paranormal life or anything.