I do not believe evil is the nature of humanity but rather that humanity is "damaged goods", to paraphrase St. Augustine of Hippo. It would be folly to limit this term to a religious definition any more than "good" is strictly religious; atheists who claim there is only "wrong doing" have probably never encountered much of pure evil, which knows no denomination. If atheists wish to restrict "evil" to a "Biblical term" than they must also abandon terms such as "love", "justice", and "compassion" as "Biblical" and therefore beneath their strictly scientific analytical tools.
Augustine's theodicy (justification of God's goodness in the face of seemingly irredeemable evil)is based upon human abuse of God-given free will, but it has its flaws. Could not God have foreseen the disasters that would follow from humanity's wicked choices"? Augustine was also weak in defining evil merely as "a privation", such as an amputated limb or lost ability such as hearing or eyesight. Evil is a dynamic and powerful force, although it is by its nature PARASITIC. It feeds off goodness and good people and seeks to destroy their work, and theologians call this destructive and self-destructive capacity in people the result of "Original Sin".
Those who wish to dismiss such "Biblical" terminology must nevertheless face the fact that human aggression is perfectly natural. Rousseau's idea of innocent Man in the free state of Nature is a liberal pipe-dream. Sadly, Hobbes was more accurate when he called life in the state of nature "nasty,brutish, and short", rather like a rude pygmy. It is fun to idealize small children as perfectly innocent, but look what they do to each other if there are no adults supervising them: they will maim, claw, and pummel one another savagely for a toy, a piece of candy, or just for the joy of it. That,of course, is only part of the survival instinct, and every adorable tot is nonetheless thinking "Me, mine, what's in it for me?" (Sadly, it seems a majority of adult Americans have not moved beyond this primitive self-absorption.)
It is society, mostly aided by religious tradition, that has curbed this savage tendency with reward-punishment based regulations, which are essential. With growth and maturity humans will come to respect the difference between right and wrong as something that effects all levels of society, and behave accordingly. A precious few will altruistically embrace doing the right thing for the right reason, and we may call such people saints, either in a secular or religious sense. But to think we are born at this third, advanced level is sheer insanity, and for all the historical abuses of organized religion, it has done more to encourage such growth than any abstract secular humanism.
Recognizing the reality of evil with a capital "E" has always been a problem for liberals and so some try to shuck it off as an outdated, "unscientific" appellation. Yet the 20th century, the most scientifically and technologically advanced period of human history,has seen more "Evil" than all the medieval abuses of the Christian and Islamic religions. It is hardly moral progress to think of Hitler, Stalin, and Mao Zedong's massacres as simply "wrong doings", yet it would be folly to say such people represent the scope of human nature.
The nature of human beings is not evil but must be disciplined to allow its benevolent side to flourish. This can be done in either a religious or secular environment, but it can never be done if we refuse to accept the existence of "evil"-ignoring the problem cannot make it go away, and all the think tanks, sit-ins, and peace marches have had no effect.
The only way to do "damage control" with human nature is to first acknowledge that it is indeed "damaged goods".