"If only it were all so simple! If only there were evil people somewhere insidiously committing evil deeds, and it were necessary only to separate them from the rest of us and destroy them. But the line dividing good and evil cuts through the heart of every human being. And who is willing to destroy a piece of his own heart?"

-Solzhenitsyn

IMO greater evil is most commonly the fruit of misconception about what is good (or necessary). I think a key part of the "battle" against "evil" is to constantly interrogate one's own ideals and methods.