The problem, as I said before, is that these faux Omicrons have the same characteristics and intentions as the originals. They were even prepared to send out a signal to the original Omicrons, which unlike the other races in Ultima Thule aren't extinct. It's noted this would have seen them resume their mission of destroying all life, which their counterparts in Ultima Thule were well aware of. In other words, they were fully and knowingly willing to be complicit in the eradication of every living thing on every world.
The Garleans mostly employed the least amount of force possible when acquiring new territory, and the people they conquered mostly got on fine so long as they stayed in line. Depending on governor, of course; we know some territories, notably those ruled by Zenos, weren't treated well at all. In any case, the Garleans weren't of one mind. Enough of them disapproved of the Empire's expansionist practices for it to be a proper political issue. At this point they have no Empire, and the numbers of Garlean purebloods have been reduced so heavily that maintaining the population is probably no longer possible. And beyond all that? They, including the ones who were against it from the start, aren't being forgiven. The story makes this evident. Garlemald is screwed.
Emet-Selch may have gotten a "fanservice sendoff," but it's made clear he and the other Ascians aren't forgiven for their actions either. It is quite possible to understand, even sympathize, with a person or persons without condoning the things they've done. Atrocities are atrocities, but in their case those atrocities were driven by something just about everyone can relate to.
Rather than just giving the Omicrons a pass right out of the gate, why couldn't it have been something to work toward? That's all I'm saying. Forgiveness rarely comes easily, after all. It's something that should've taken time and hard work.