 Why does Thig leave a note at Torp's desk?
Here he patiently sat down to wait for an Earthman or an Earthwoman to pass. Feebly he had punched a key two days ago and a $ sign had appeared. "For the good of the Horde," Thig intoned almost piously as he lifted Terry's body and headed for the laboratory. An hour later, his scars and altered cartilage already healed and painless, Thig again scraped sand over the entrance to the space ship and set out along the moonlit beach toward the nearest path running inland to his home. Men had no mates on Ortha, sex had been overthrown with all the other primitive impulses of barbarism; so he was incapable of understanding the emotions that swept through his acquired memory.
 He wants to make sure no one comes to invade Earth, and have reason to fear doing so.
 He wants to make sure no one comes to invade Earth, and have reason to fear doing so.  He wants to warn the other Orthans about the potential dangers of Earth.  He wants someone to understand what had happened.  He feels badly about killing Kam and Torp, and wants to leave a final message on their behalf.