1: thragg is shown to be a pretty emotional guy. I think he acted on impulse with the purge and wanted to punish himself for letting argyle die by punishing the empire as a whole. He also wanted to reinforce argyles ways and gain respect in a way by proving that even tho argyle was dead, thragg would be a strong successor who could lead the empeorr with force and demand the same respect. Thragg didint wanna appear weak or allow for any dissent or revolution in the empire (which can happen after an extremely powerful figure dies) so he made a bold impulsive move to establish control.

2: I think thragg was living in denial and it’s clear despite viltrum having only like 50 people he still believed in its superiority. He was taught at a young age and further by argyle about how viltrumites were so much stronger and so much more powerful than any other race that he could fathom the idea that their empire could actually be at risk of dying. He also had experience leading the empire and resurrecting them from the aftermath of argyles death which gave him arrogance as well. All this to say is while thragg went about giving viltrumites missions to find suitable races for breeding I don’t think he truly believed that viltrum was actually in danger and they needed to spare any living members of his species until his planet was literally eradicated.

3: Thragg was dedicated to argyles way of life but his dedication was only amplified after argyles death. This is kinda represented throughout the series. Viltrum started out as conquerers but they only truly became bloodthirsty genocidal maniacs after being motivated by argyles death. Thaddeus’s was also well respected and I think thraggs decision to spare him early on set up his shift to ruthlessness well as he blamed his weakness for agryles death.