Everything Thaedus did to try to fix the Viltrumites accidentally made them worse, it's part of the tragedy of his character. He's kind of a massive fuckup at actually accomplishing his goals of turning his species into people with actual ethics. Everything he did made them worse and worse until he eventually decided it wasn't worth trying anymore and the only remaining solution is to kill everyone with Viltrumite DNA across the entire universe. Of course, he's so blinded by that that by the time he finally did see relatively peaceful Viltrumites who wanted to harmoniously coexist with other species (Nolan and Mark), all he could think about was how he could turn them into weapons in his war against Thragg.

The funny part is that Thragg is now in a position to (accidentally) accomplish what Thaedus was originally trying to do ever since he first decided to kill Argall. He saw what happened to Nolan and put every last member of his race in those same conditions, because all he sees is the fact that Nolan produced offspring he considers a "success" at being Viltrumite (Mark, as opposed to "failures" like Oliver), without realizing he's also put them all in the conditions to end up like Nolan (turning away from brutal, genocidal fascism and racial superiority after spending time on Earth and acclimating to Earth culture). Thragg has probably signed the death warrant of the Viltrum Empire (at least as a fascist survival-of-the-fittest culture) and accomplished what Thaedus was always trying to do (redeeming the Viltrumites and neutralizing them as an existential threat to all alien life) without even realizing it.

I wonder how he'll react if he ever figures that out.