People are neither inherently good nor inherently bad. What they are is inherently self-interested. To the extent that people's perceived self-interest coincides with what is good, people are good. To the extent that people's self-interest coincides with what is bad, people are bad.
This is a key point which liberals completely fail to grasp.
I believe in the concept of original sin. Man is born into sin and cannot free himself. It is more than self-interest. People engage in all manner of evil acts that will likely be discovered and cause all manner of pain and punishment to themselves and others. Some engage in murderous acts and willfully commit suicide in the process. The wholesale gassing of Jews in concentration camps falls outside the envelope of human self-interest. Stalin, Pol Pot, Mao and Saddam Hussein engaged in a level of evil far beyond such explanations.
The term "self-interest" simply does not cover the depravity of human conduct.