If I may Bella, their behavior was erratic true they were all heavily flawed individuals and the Dr knew it she was a psychiatrist. They were chosen for that very reason. The Dr had no intention of anyone returning because her flaw was cancer she was dying and had nothing to lose. Also the movie was poorly edited as well because some important info was left out from the the final cut. The title of the movie for instance was the control word used to delete memories from the women should the need arise and apparently it did. Do you remember when they woke up with no memory of setting up camp and there had been more food eaten than they could account for. That code word Annihilation had be used but it had been cut from the movie. I loved the concept that a life form with no true concept of itself needing only to find a way to survive by altering the life around it until found a suitable form. There was no intellect there until it found it’s awareness in Oscar Isaac’s character. It had found awareness in the others but their flaws made them unsuitable because they were not on a course for a sustained existence. While Oscar was dealing with the pain of betrayal the alien found a means to survive through his love of Natalie Portmans character. He had no intentions of continuing the relationship with her so his form was copied with out the memory’s of her betrayal. Her flaw was her guilt for betraying him but her love for him was what drove her. So she was flawed but the flaw didn’t hamper the coupling in fact it was why she didn’t need to be replaced. So finding it’s best way to survive in them It ended all other attempts of finding a form to continue living. I thought it was pretty cool myself, even though the movie had it’s issues.