This might square with a reality where people are fully in control of their development throughout their lives. However, we know for a fact that human beings are profoundly shaped by deterministic factors.
Factors like upbringing, where we're conditioned in our formative years by parents, peers, authority figures, and personal experiences. If you're beaten a lot as a kid, or you lose a parent when you're young, or you're repeatedly humiliated when you express joy, that changes the course of your development -- how you feel, how you think. You might find yourself overwhelmed by rage and disgust toward any form of authority. You might be subconsciously motivated by a deep seated fear of change or loss. You might hate yourself.
Other determining factors spring from our genes. What about mental predispositions that are totally out of our hands, like being prone to anger, anxiety, and depression? How about being born with a physical handicap? Different people handle adversity in different ways. It's not exactly a stretch to see how someone's life could be defined by pain, shame, or any kind of social obstacle.
These aren't excuses for being a bad person; we should all strive for self-improvement. Rather, they're factors outside our control that have a demonstrable influence on thought and behavior.
We don't all naturally drift toward decency. We're not all playing with the same hand, with the privilege of good parenting, strong and progressive social ties, an even emotional keel, access to therapy, an adequate education, or the resilience to break our programming.
To ignore all this in favor of a more satisfying narrative of radical personal responsibility is, well, less than reasonable. Assigning blame in stark black and white terms is not reflective of the empathy and understanding we would hope to see throughout society.