We are born innocent as babies. It is good in nature.
But we are exposed to different environmental conditions as we grow up. Depending on the surrounding environment that will lead to be good or turning the evil side. It is what we see and tend to follow the same suit.
When we see many good things, then we will be doing lots of good stuffs. Only when we encounter evil things then it will be up of our hearts to decide right from wrong. It is our upbringing that will be put to a test. For those who have been doing good and having positive thinking, it will likely not affected by the evil we see.
Between good and evil, it is what we decide.