Mere Christianity: pure evil?

Mere Christianity is a great book. Lewis has a way of putting things in a very reasonable way so that if I were not Christian already I might be persuaded to become one. But because a bunch of "I agree's" do not a web log make, I will put a mix of my strong agreements, my added thoughts, and my objections here, because that makes it interesting.

As you read these objections, just remember that between each idea that I object to are many, many sentences that I agree with. Overall, I agree with nearly everything Lewis says. But also realize that the following objections are not a comprehensive list of everything I disagree with. Just the ones I chose to write about.

On page 44 (book 2, chapter 2), C. S. Lewis asserts that

"No one ever did a cruel action simply because cruelty is wrong--only because cruelty was pleasant or useful to him."

I disagree. I believe that men can become so satanic as to do things for only the sake of evil. The example I can think of now is that of a gang bent on hatred of some race of people. In some real gangs, entrance is granted to those who have killed someone in the hated race. Now the man who is in charge of that gang, who invented that rule, stands to gain nothing from that rule. He gains no pleasure from a killing he had no part or knowledge of. Yet because it is wrong he does it. And it strikes me that there is no other reason.