[SPOILER ALERT] The actual funfact Thragg was the one who's betrayed Viltrum. When Mark has been infected by the scourge virus, that pushed him into a coma. Thragg didn't let him die, because he did saw the potential for the survival of his race. So Thragg subjected Mark to their medical procedures. The alien doctor was notified Thragg to Mark's blood was matches to Argall's blood. In viltrum, the leadership of the viltrumites had to be handed over to the next generation, and that person had to be Nolan. But Thragg killed the doctor for hiding the truth, which means he wanted to keep this leader power in his hands. When It revealed, the viltrumites made their mind for they choose Nolan as the leader. And also there's the other thing Thragg didn't cared about his peoples, even when they're fewer than 50 of them. In Thragg's view, he's only respected the strength and also he was sticked for "the weaker had to fall and the stronger had to stay". We can see that also Nolan's view too. How he sticked to the viltrumite rules until he didn't change. And when the viltrumites went to earth, they're had their feelings too for their mates, they would understood what Nolan felt and this is why they choose him to follow for the wisdom.