I see this argument boiling down to two trains of thought. One is that people are fundamentally good and only external forces make them do bad things. That appears to be the opinion you have expressed.
The second is that people are fundamentally bad and that external forces are required to keep them from doing bad things. I think the more you look at the world the more you find this second postulate is the right one, and your looking in the wrong direction for justification.
Reality is not so dichotomized. Human choices are influenced by both nature and nurture (i.e. environment). In psychobiology, I believe the consensus is that nurture contributes at least 50% to personality development and consequently propensity for certain choices and behavior. If inherent nature can't be changed, how could have the environment been different, to change the outcome? That was what I got from the parent comment.
By the way, if we're talking about "fundamental" qualities of a person -- from a biological standpoint, there is some evidence for a genetic propensity toward 'good' (e.g. altruism) or 'bad' (e.g. violent) behavior. But the vast majority of people land somewhere in the middle.
A third train of thought is that we evolved to live in groups, able to show great empathy--the source of most "good" actions--for members of our group, and no empathy at all for anyone else. That would acknowledge our natural tendencies to be helpful and loving, selfish and destructive, depending on worldview and circumstances.
The second is that people are fundamentally bad and that external forces are required to keep them from doing bad things. I think the more you look at the world the more you find this second postulate is the right one, and your looking in the wrong direction for justification.