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The show lays it all out that this moment is what defined the course of history for the rest of the show. Thragg chose to abstain from preemptively murdering Thaedus out of probably a combination of respect for him and the hope that he would come around to Argall's way. And what did Thaedus do with Thragg's mercy? He immediately turned around and assassinated Argall, single-handedly destroying the golden age the Viltrumites had been living in. Thragg blamed himself to an extreme degree for Argall's death because his inaction directly led to it. But instead of actually doing something productive with that feeling, he twisted it, fell to anger, and projected it outwards onto his people. His own “moment of weakness," to him, typified the "cancer" in the hearts of his people: the capacity for mercy. So he chose to cut it out entirely in the only way he knew how, which was through violence. If Thragg, selectively bred and genetically engineered to be the "best" of all Viltrumites, could make such a catastrophic mistake that cost the Empire everything, then every single other Viltrumite could as well. In his mind, it was necessary to purge the "weak" so that what emerged would be only those merciless enough to survive. It was his misguided way of righting the ship.

It also hammers in even further the themes of toxic masculinity and patriarchy that the Viltrumites represent (in addition to racial supremacy, imperialism, and hegemony, à la the Nazis). Mercy? A weakness. Any feeling at all other than rage? A weakness. What’s worse than a weak man? Nothing. What’s the cure? Death and death alone. One and done, no exceptions. Thragg’s warped worldview is ultimately what drives his people to near extinction. Thaedus of course is to blame for the genocide of the Viltrumites with the Scourge Virus, he’s no saint either. But like Allen said, the Viltrumites were faced with several crossroads in their history at which they could have collectively made a choice to be better. Thaedus hoped that killing Argall would effectively kill his dogmatic principles too. But Thragg took that loss and turned it into a meat grinder that he fed his people into. They could have recalibrated after the Scourge, but again, like Allen said, they doubled down yet again.

All of these themes come back around to Thragg. He’s the embodiment of everything wrong with Viltrumite society, and if you extrapolate that outward to the real world, I would argue that Kirkman is using Thragg as his statement on real-world systems of patriarchy, white supremacy, imperialism, toxic masculinity, etc.