 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. Then there would remain only the task of blasting the great, shining ship out into the carefully-calculated orbit that would be its home. Out there, away from the muffling, distorting, damnable blanket of atmosphere, away from Earth's inexorable gravitational pull, would be a laboratory such as man had never seen. "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. They have examined us, dissected us, driven us mad, killed us with no discernable provocation; and this is more intolerable than any normal invasion. "I have no fear that the people of Earth will fail to meet this challenge, for there is no alternative. Not only our individual lives are threatened, but our existence as a race. It was only two stories down the moving ramp to Lorelei Cooper's laboratory. Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation." There were left only two blood-red, malignant monstrosities somehow defiling the air they floated in; and below them, a pitiful huddle of flesh no longer recognizable as human beings. There aren't going to be any noncombatants, this time.
 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