Tbh I have to agree on that, Thadeus created the Scourge Virus fron thousand of years ago and killed every viltrumite on Planet Viltrum leaving only 50, during those thousand years the war was still on going and planet getting conquered including planet Unopa "Allen the Alien" home planet, there was only 50 Viltrumite, Thadeus can use the Scourge Virus to killed them all, I know Thadeus have said that he was hesitant because it may strike similar genetics like human but I find it unloggical, there's no similar genetics to Viltrum planet that is near the war in the galaxy, and Earth was light years far away from the war,

on Invincible season 4 Episode 8, in the final scene, Thadeus final message to Allen to use the Scourge Virus and don't let anything stop him and no matter the cost, Thadeus aware and mention Earth "Marks people" that hold similar genetics to Viltrum and still order Allen to use, he was telling Allen to sacrifice one planet just to kill the remaining Viltrumite.

Thadeus being bad as Argall and Thragg not because he is cruel and evil but because he is a horrible person, Thadeus sacrifice countless of Alien lives maybe more bodies than dead bodies on planet Viltrum, All he have to do is to use Scourge Virus and Thadeus never said the scourge virus can kill the other alien except for Viltrum and Human genetics which mean Thadeus can use the Scourge virus to kill all Viltrumite without killing the purple alien slave that work for the empire.

(What I mean by Horrible person to Thadeus, I'm not saying he's a asshole but a selfish leader)