 Why does Thig change his mind about the invasion?
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. An expeditionary force to wipe the grotesque civilizations of Earth out of existence would, of course, be necessary before the first units of new Hordes could be landed. For three months he had lived with people who loved, hated, wept and sacrificed for reasons that he had never known existed.
 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.