Suppose one reads a story of filthy atrocities in the paper. Then suppose that something turns up suggesting that the story might not be quite true, or not quite so bad as it was made out. Is one's first feeling, "Thank God, even they aren't quite so bad as that," or is it a feeling of disappointment, and even a determination to cling to the first story for the sheer pleasure of thinking your enemies are as bad as possible? If it is the second then it is, I am afraid, the first step in a process which, if followed to the end, will make us into devils. You see, one is beginning to wish that black was a little blacker. If we give that wish its head, later on we shall wish to see grey as black, and then to see white itself as black. Finally we shall insist on seeing everything — God and our friends and ourselves included — as bad, and not be able to stop doing it: we shall be fixed for ever in a universe of pure hatred.This is at the end of a chapter about loving one's neighbor, even in the eventuality that the person is a real enemy. Lewis, of course, had Nazis freshly to mind. We have ISIS and the like to consider. He gives very helpful examples about how to come to grips with loving the sinner while hating the sin.
C. S. Lewis, Mere Christianity
I hope, unlikely to come across a member of ISIS. I am, as we all are right now, very likely to come across someone who passionately supports a political candidate or opinion I despise. Keep that in mind and then reread the quote above.
Here is the key bit and one which I was shocked to realize I recognized in myself: "is it a feeling of disappointment, and even a determination to cling to the first story for the sheer pleasure of thinking your enemies are as bad as possible?"
It is then that we realize, as Lewis puts it later, that "hatred [is] such a pleasure that to give it up is like giving up beer or tobacco."
It is that we must fight.