One thing that is called a necessary evil:
--the war in Iraq
Are there really necessary evils? As Tolstoy remarked in his "The Kingdom of God is Within You", where do you draw the line on killing someone? It has been said that it's been a necessary evil to wage war on Iraq in order to bring the citizens to freedom and a better government. It has been said that it is a necessary evil for civilians in various countries to be killed in order for the better good for the survivors.
Tolstoy talked about drawing the line on who you would kill. So you're a solder and you're commanded to kill the stranger for the better good. What if that other person were one of your own "peeps"? If that were your relative would you do it? (brings to mind the nightmares of the Civil War, when brother fought brother, but that's another whole set of thoughts to get into).