 Why does Thig change his mind about the invasion?
"For the good of the Horde," Thig intoned almost piously as he lifted Terry's body and headed for the laboratory. Twice, with subtle drugs they restored pseudo-life to his body and kept the electrical impulses throbbing from his tortured brain, but after the third suspension of life Thig removed his helmet. 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 remembers Ellen and the love he felt, and doesn't want to leave.
 He remembers Ellen and the love he felt, and doesn't want to leave.  He has forgotten why he lives for the Hord.  He contracted a disease while on Earth that's making him make wild decisions.  He is fearful that Earth's influence will affect the Orthan as it did him.