Here is a nice example of a blog post created by a student last semester
In the above quote said by Ernest Hemingway, he seems to suggest that humans are born with an inherent disposition to being evil. This makes me think of William Golding's novel, Lord of the Flies that tells the story of a group of British boys who get stranded on an island and became savages within a few weeks. 
While reading this novel in my English class, we discussed the hotly debated topic of nature (Do all humans have an inherent evilness?) versus nurture (Was it because of society, that these boys thought it was okay to steal, lie, and kill to stay alive?). At the end of reading the novel, we concluded that humans are born with a "beast within us" as Golding put it.
But as our psychology textbook states, there seems to be no clear cut way to separate nature from nurture.
During my senior year of high school, I watched a family friend's baby grow from a baby into a toddler. As a baby, when she didn't receive what she wanted she would cry and pout. But as a little toddler she would hit me when I wouldn't give her an extra cookie or something else that she wanted. This made me wonder how a little toddler barely 2 and half years old knew to hit me when I didn't give her what she wanted and certainly her parents wouldn't have taught her to resort to violence when she didn't get what she wanted.
Does this mean that no matter how good, innocent, or young a person is deep down there is a "beast" within all of us just waiting to emerge?
You raise an interesting point Yantang. Philosophers and statesman have argued for years that mankind has an evil side and that left unchecked many of us would do harm to our neighbor.
Lately others such as Steven Pinker in his latest book "The Better Angels of Our Nature: Why Violence Has Declined" argues that it may be adaptive to exert aggression when necessary in order to survive, our more day to day existence depends on the trust we build with others.
A highly functioning society is governed by laws and is at peace with its neighbors. Even though we can be imaginatively cruel when under threat, I believe that we are evolved to connect and assist one another for the benefit of all.