 Why did Robert choose to not return to Earth after Peter had told him that he was ready?
I do not think it was what Peter called an emotion; and yet it had nothing to do with logic, either. I did not even wish that Peter were not dead, for reason had told me that he had to die. "Since the Invaders first appeared in Wood River, Oregon, 24 hours ago, they have not once acknowledged our attempts to communicate, or in any way taken notice of our existence as reasoning beings. "I have no fear that the people of Earth will fail to meet this challenge, for there is no alternative. Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation."
 His logic wouldn't allow him to fulfill the purpose
 He wanted to stay with Peter, alone.  His fear of the Invaders after hearing the story from Peter's diary  His logic wouldn't allow him to fulfill the purpose  He couldn't decipher the difference in killing the humans and the Invaders