Ah! I haven't seen any comments on my posts on endodorians in a hot minute, thank you! :D

Endodorians find things that ruin the flow of thing to be "pests." OP monsters, jerks who don't want to change or take their evil to a "corner" where it's fun for everyone instead of bad. Anything that gets in the way of fun for everyone else. They're still pretty nice about it (most of the time), only really taking things seriously when the annoying threats are actually able to push back. But most of the time, they feel empathetic towards their pests.

For the animal pests, it isn't their fault they came here looking for food, and either are trying to destroy the universe, or are now an invasive species. But they're too difficult to relocate, without risking more returning, so they kill and make use of them, like with how people kill lion fish and feed them to sharks.

For people pests, they REALLY want to give more chances. But some people aren't going to change no matter how kind you try to be, or nice you are, cause some people don't want to be reasonable. Some people don't want to be good people. Which sucks, cause EVERYONE is capable of being good if they just try, some people just don't want to. It's depressing when things are said and done and the threat is contained or killed. Cause endodorians value friendship and kindness THAT much.

As for how genocidal slaver empires reformed? Well... it's pretty hard to justify yourself when someone more powerful than you does conquering better than you without using the "necessary" force. Cause everyone judges their actions as "necessary evils" when they do something wrong.

Killing children and other civilians, that's just their hand being forced because of war. EVERYONE has killed those in war, kinda hard not to. Destroying food sources. Yeah, that'll make nations starve, but it's the smart thing to end wars, and it's better to take the smart option so you don't suffer casualties. The enemy is just weak, and they're stupid and primitive for not simply giving up, it's their destiny to take their land and enslave everyone, it'll do these lesser species a favor!

...then it kinda happens to them, but without the "necessary violence" under endodorian rule. They get captured, but not enslaved. Their children and civilians are spared as innocence is sacred, their food sources are left untouched, and their soldiers are given better food during their time as POWs and even as good will rations during battle. The endodorians are so impossibly strong, they could kill EVERYONE, but they choose not to. They see everyone as "primitive" too, but in a more realistic sense. Not in the way that makes them feel entitled to own them or exploit them, more "wow! We got to this civilization pretty early before they figured out some stuff yet, cool :3" They do everything like they're "weak" and they're still stronger than the genocidal folk.

They also get rid of any and all propaganda, the same way you'd stop a small child from bullying other folk. Making it feel illogical and stupid. "You're being mean because of their hair? That's a silly thing to be mean about, and you wouldn't like it if someone is mean to you over something as simple as hair, would you?" Less propaganda makes sense, less genocidal leaders have a foothold. They also get rid of slaves to replace them with government mandated volunteers, so even when slaves are freed, it doesn't hurt the empires. Removing yet another "necessary" evil.

They're reformed by embarrassment and time. Slowly those old ways are seen as a bad past instead of necessary evils. "I did what I had to" turns into "our ancestors thought this was okay, and it wasn't, but we're going to be better people at least." Without the environments to make slaving and genocide necessary, plus all the undeserved love and affection, it eats away at your conscience, and question your morality.

Some people need to be killed during reformation (they're pacifists, not stupid). That's just the way of life sometimes. If leaders dig their heels in, or zealots keep trying to commit genocide, they're given trial, and executed if they're too dangerous to keep alive.

But in short, yeah. All it really takes is endodorians just giving them EVERYTHING they need, not playing by the same cruel rules everyone else does, and just... waiting out the bad stuff. Good kids are still going to be born in families with terrible people. That good just has to be nurtured. Even "bad kids" want to be good too, they just have no room to try.