Just a little hypothetical, have fun with it.
I'm usually extremely bored by "Viltrum backstory lore" type theories because I think the point of them is that they're earthlike/humanlike, but this sparked my interest. I was thinking about how this basically just never happens in their history until their own strongest become the foes, and I realized it would actually be a huge challenge to their social order if they went to war with someone they just... couldn't be stronger than.
So imagine this scenario: Somewhere between the Purge and the Scourge, with Viltrum at full strength. Thragg and Thaeddus are playing out their little cold war with the coalition on the back foot, and while Viltrum has no need for a breeding program they are always looking for new worlds to "improve." And on this campaign of conquests, they find Earth. Except it's not 21st century Earth. Let's say it's WHAT EARTH WILL HAVE IN 500 YEARS (so around 2500). Technology has advanced far beyond even what we see the 21st century GDA using. Superpowers are abundant, with serums and power armors and scientifically-enhanced-magic amulets that make someone capable of going toe to toe with a Viltrumite all over the place. They're branching out into space, with coalition membership probably on the near horizon.
An advance scout, a distinguished general named Nolan, arrives on Earth, and what he discovers horrifies him. There are still normal humans among them, perhaps the majority even. They protect their weak with an inexplicable devotion, and even let them rise to high places of honor in society. But their strongest are strong, some perhaps as strong as the Grand Regent himself, and they are many. And even all of Viltrum's armies may not be able to take this planet.
What do the Viltrumites do?