In addition to reading religious philosophers such as St. Thomas Aquinas, I enjoy reading some of the anti-religious philosophers. One of my favorites is Ayn Rand. Most people know her from her books such as The Fountainhead, Anthem and Atlas Shrugged, among others. She is less well known for her philosophy of Objectivism but anyone who has read her books (and even many who have not) are familiar with it. While I enjoy reading her philosophy I should add that like any other philosopher I cannot agree with everything she said or wrote.
One of the reasons why I like reading her philosophy is because she has very good concepts regarding morality. One such idea is her concept of evil. As explained by Leonard Peikoff in his book Objectivism: The Philosophy of Ayn Rand:
"Evil is not consistent and does not want to be consistent. What it wants is to inject itself into the life-sustaining process sometimes--short-range, out-of-context, at whim. To achieve this end, it needs only a single concession by the good: a concession of the principle involved, a concession that evil is proper "sometimes." Such a compromise is evil's charter of liberty. Thereafter the irrational is free to set the terms and spread by further whim, until the good--and man--is destroyed.
"The power of the good is enormous, but depends on its consistency. That is why the good has to be an issue of "all or nothing," "black or white," and why evil has to be partial, occasional, "gray." Observe that a "liar" in common parlance is not a man who always, conscientiously, tells falsehoods; there is no such creature; for the term to apply to a person, a few whoppers on his part is enough. Just as a "hypocrite" is not a man who scrupulously betrays every idea he holds. Just as a "burglar" is not a man who steals every item of property he sees. Just as a person is a "killer" if he respects human life 99.9 per cent of the time and hires himself out to the Mafia as an executioner only now and then.
"To be evil "only sometimes" is to be evil. To be good is to be good all of the time, i.e. as a matter of consistent, unbreached principle."