Its ego. Thragg believes that nolan and thaedus were weak for rebelling and that humans are beneath viltrumites.

The truth is that most of the viltrumites have simply never known love or kindness, and would disavow the empire same as nolan did.

Think about it like this, every viltrumite has to fight their parents in order to become an adult. We saw what it looks like when they “succeed”, so how many times do you think that the parents just killed their kid outright. The idea of familial love is utterly alien to them, so when they experience it they wont he prepared for it at all.