Morality isn’t hard-wired in humans. Its a social construct. Besides the fact that “good” and “evil” don’t truly exist, people do “good” actions because it most benefits them. Humans are a social society and we often need each other to survive. Going back to thousands of years ago, if there were a group of other people who stole food from your people’s land, the most logical thing to do would be to kill them, as they do not benefit you or your people. Today, we have laws to prevent such things, and people live by the order of these laws.
All “evil” is, is disorder. Chaos. While “good” is order and following the rules. It has nothing to do with “morality” or anything else. People will kill or murder if they have to, such as going to war or hunting down Bin Laden, yet still think of themselves as good people. Human beings can also have remorse in the most minor of actions, thinking of themselves as “bad” people for doing things that haven’t really hurt anyone, such as sleeping with a lot of people or acting in a way the community would not approve of, even if no one is hurt in the process.
Humans have no innate morality in them. All “good” is, is cooperating with others so we can survive. All “evil” is, is disobeying the law and order of the land. It is merely survival as it is much harder to survive life on your own. That’s why we live in communities and have grocers and garbage men and politicians and sales clerks, to make human life and survival much easier. Things like killing and stealing go against the order to make a proper community run. So we are taught not to do these things from a very early age. This isn’t even getting into different societies and things they do which would be considered “right or wrong” in other places.
There is no such thing as “universal morality.” Anyone who says so is either an idiot, a religious zealot, or both. We merely do what we have to do to survive. And well, raping and killing kind of makes it hard to get along and survive in a community together. That’s why post-apocalyptic movies are so interesting. When society collapses, so do the rules that held it together. When Rick Grimes shoots a group of marauders who want to raid and kill his group, is he “evil” for committing murder?
As @poisonedantidote said, humans beings are not good nor evil. No more so than wild animals. Human beings are merely selfish. Even in “civilized society,” few care about others outside their own family and social circles. This is also what dooms humanity and makes them closer to “evil” than “good.” As goodness is seen as cooperation, togetherness, and coming together to achieve more as a society than we ever could as individuals. However, evil is typically defined as selfish actions that do great harm to others at worst, and is indifferent of them, as best. Sadly, that is human nature.
If you want a simple answer: humans are inherently “evil” because most all of them are inherently selfish. And selfishness is the primary motive of evil-doing. Any acts of “goodness” usually come from either survival or guilt of inaction, at the most. People rarely do “good” things without benefit or gain from them. This is what makes “goodness” so admirable and highly esteemed. Other than it benefiting others, it is hard to be good! It involves sacrifice and putting others before yourself. Most things that are hard to do are held in high regard in society. Goodness (as in, cooperation and helping others) merely has more positive benefits in a society than negative benefits. While evil (as in, selfish behavior and living life lawless) is often detrimental to society.
Humans have had a good 5,000 years to get accustomed to this, so its no wonder ‘goodness’ and democracy is the way we are taught and ‘evil’ and anarchy is to be feared and rejected.