Are human beings born good/evil?

I was currently reading about the question wether one human being is already born as an evil one or just develops to this kind of state because of the environment the person is confronted with. What is your opinion on this topic?
Does anyone know interesting links/books etc. about that question or similar themes?
I personally think it's incredibly hard to answer this question with yes or no. Because on one side there are those who live in bad situation in family and bad area, where they learn wrong things and get to meet the wrong people and other children without any of these experiences turn out to be cruel from a really early age on without any known influence.
Do you even believe in this seperation of good/bad? I think the huge part of humans is in the grey part, in the middle, because there are no perfect humans, who are always good, no matter what situation. And other question: what is the definition of good and bad for you?

And do you think people can make experiences that make them want revenge, because they got hurt or someone harmed them or family?
Like, could a stranger spoil one human being by inflicting great pain, physically and psychologically, upon him? The pain turns in anger and the person, that got hurt, does not forgive nor forget?
What do you think or have experienced/heard?