I mean, ideally you'd have turncoats, folks who aren't buying into the regime, various other people and whatnot joining up with the other side for power, fame, fear....
But, you've found the solution. Regimes be evil, races aren't. Plus, folks remember and like the stories of people driven to fight each other through circumstance, through who had things been different they may have been friends. No one cares about Orphanageburner the Bandit who burns orphanages because he's Chaotic Evil and your players will reduce him to a bloodied smear on the ground in 3 rounds, tops