 Why does Thig change his mind about the invasion?
Thig was shorter than the average Earthman—although on Ortha he was well above the average in height—but his body was thick and powerfully muscled. Here he patiently sat down to wait for an Earthman or an Earthwoman to pass. His task now was to bring a native, intact if possible, back to the carefully buried space cruiser where his two fellows and himself would drain the creature's mentality of all its knowledge. In this way they could learn whether a planet was suited for colonization by later swarms of Orthans. Already they had charted over a hundred celestial bodies but of them all only three had proven worthy of consideration. This latest planet, however, 72-P-3 on the chart, appeared to be an ideal world in every respect. Feebly he had punched a key two days ago and a $ sign had appeared. "With a psychic relay we will transfer the Earthman's memories and meager store of knowledge to the brain of Thig! He can then go out and scout this world without arousing suspicion. "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. My new body must return to its barbaric household before undue attention is aroused. 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. Tonight or tomorrow night at the latest he must contact his two fellows and report that Earth was a planetary paradise. No other world, including Ortha, was so well-favored and rich. 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. There was no abrupt division of men and women into definite classes of endeavor. Uncertainty added zest to every day's life. For a hundred thousand years the Orthan civilization had remained static, its energies directed into certain well-defined channels.
 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.