Why must people be good or bad? It is a rare person who wakes up in the morning and decides to set out to make the world a worse place or to destroy the lives of billions. Good people are capable of fighting each other, quite passionately, particularly when they are unable to see the reason behind their actions.
Propaganda often tries to homogenize the public perception. "We" are just. "They" are evil. While this may be true - it is most often a tool to convince the public to support a war against an incomprehensible evil rather than to see the opponent as having a set of objectives to be reasoned with.
To use a lord of the rings analogy, Sauron was the epitome of evil, insofar as he was willing to crush all opposition and tolerate no dissent. While he no doubt rationalized his own actions, his diplomacy was more or less "convert or die." And as such any sane person would fight against that encroaching horde, or at least move out of its way if possible. It was the weak and/or fearful who joined Sauron in the belief resistance was futile and that it was the best course for their people.
And then Sarumon thought he could snatch the One Ring away and use it to rule, himself - his goal of opposing Sauron becoming twisted into a plot against middle earth.