 Why does Thig change his mind about the invasion?
He had blasted across trackless space to subdue a defenseless world—only to meet on Earth emotions that were more deadly than weapons. He stared appraisingly around at the luxuriant green growth of foliage further inland; and started toward the little stretch of trees and brush, walking carefully because of the lesser gravitation. 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. Then he had no time for further speech, the massive arms of the stranger had wound around him and two hamlike hands shut off his speech and his wind. "There it is," announced Thig, dropping the limp body of the captured Earthman to the metal deck-plates. "Thig will be this creature!" announced Torp. "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. "Do not question the word of your commander," growled Torp, swelling out his thick chest menacingly. "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.
 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.