Evil is "good people" thinking "the ends justify the means".
"It's unfortunate we have to persecute these even the women and children, but it's required to make a more fair and just world."
"It's unfortunate we have to go to war in Iraq, but it's required to defend peace in the region, from the threat of mass weapons of destruction."
Of course, "the ends" are never achieved, because the "means" they use will cause even more problems to be solved.
Persecuting Jews to achieve justice and fairness for the German people => Destruction of Germany.
War in Iraq to remove dictatorship and mass weapons of destruction => Rogue terrorist group governing by coercion and hundreds and thousands of civilian deaths.
I ask you. What other examples of evil can you see in our world, today? What other evil things are people are doing in the name of good?
Most important to remember: Evil is stopped by being persistently good ourselves, even when pressured and coerced to do otherwise, and that's all that we have to do.