 Why did Thig react with violence towards Kam while they were traveling back to Ortha?
The Orthan entered the narrow strip of trees and crossed to the little-used highway on the other side. 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. 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. "With a psychic relay we will transfer the Earthman's memories and meager store of knowledge to the brain of Thig! "For the good of the Horde," Thig intoned almost piously as he lifted Terry's body and headed for the laboratory. So it was that the bodies of the Earthman and the Orthan were strapped on two parallel tables of chill metal and the twin helmets, linked to one another by the intricacies of the psychic relay, put upon their heads. 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.
 He wanted to return to Earth and to Ellen.
 He wanted to return to Earth and to Ellen.  He did not want his blood tested for disease.  He was angry that they had killed Lewis.  He did not want to live on Earth any longer.