No one has to teach a small child to lie, to steal, to be selfish etc. Those things come quite naturally to everyone.
Having worked a lot with kids I agree with that one. It's astounding how morally twisted a little child's logic can already be; my favourite one is the child that after being told that sharing is a good thing comes up to me and tells me that he really should get some little girl's toy because "she has to share". Using good and selfless rules to serve your own selfish motives, you don't have to teach that to a child.
Living in a tough big city I am also not that idealistic about human nature, people can be very selfish, me included. On the other hand I have also come across truly selfless, genuine acts in the last few years, so that's the other side of humanity. As a Christian I agree with Fidelia that we are inherently bad but still created in God's likeness, which gives us an ability to show compassion, be fair, try to do the right thing etc. However after studying history (especially Russian and German history are great at showing what amounts of evil people are capable of) I am regularly thinking that I don't want to be in a pressure cooker situation like a war where everything that is in people's hearts comes to the surface, I am not sure that would be a pretty sight. (On the other hand for example after the Japanese tsunami the Japanese seem to have behaved in exemplary, calm fashion so that shows the good in people again.)
Actions can be 'good' or 'bad', but people are just people.
Besides, what kind of knowledge would we gather if we could actually determine if somebody is 'good' or 'bad'? What would be the consequences of that? The person would still be the same, just that it would come with a label.