There is an old expression that goes: "evil is what happens when good men do nothing." This is utter hogwash--when good men do nothing, what we end up with is a typical Sunday on the couch. Sloth, my friends, is what happens when good men do nothing! Evil requires some evil guy doing something. Something evil.
Now, good men doing nothing sure makes it easier for evil to happen. But then there's the question as to whether you're still good if you did nothing while knowing that evil was happening. Sitting idly by while Hitler massacred Jews and other innocent folks during WWII was one of the things that made FDR and Churchill less good--not to mention doing nothing while Stalin did his share of evil before, during and after the war. So maybe it's more correct to say that evil requires some evil guy doing something evil, and weak guys letting it happen.
So where are the good guys in this? Well, if you're good, and evil happens, it's either because you don't know evil is happening, or you are good but powerless. So the new equation goes as follows:
Evil = Evil guy doing evil stuff + weak guys doing nothing + (good guys + ignorance or good guys + powerlessness)
Of course, George Orwell probably wouldn't want to try and fit that on a poster.