The old notion that babies are born a blank slate is not true, per the research. Infants come into this world with an array of temperamental qualities, which leads to a different personality constellation into adulthood, which affects adult behavior and emotionality. With that said, we do know the brain is wire hard for altruism and cooperation, which has been supported by numerous studies within the field of neuropsychology, psychology, and neuroscience.
Of course, a specific temperamental disposition will lead to a certain personality constellation, which may increase or decrease the chances of someone engaging in violent acts. Of course, the environment can act as a mediating or moderating variable, as the research is quite clear that linear models of causality are rather "outdated" and there are multitude of variables that will determine if a person can commit heinous acts toward their fellow man.
In EXTREMELY severe cases, I do think that most people would engage in acts that would promote the survival of themselves or their loved ones over others. However, I think there is more of a middle ground on the spectrum, where is is really difficult to say what particular variables may or may not lead to an individual engaging in evil acts.
The fact is, most of us doing have our breaking points--we all have a psychological threshold, that if pushed over the edge, could lead us to possibly engage in a variety of behaviors--violence, psychosis, substance use/abuse. The arguments here are too restrictive in nature--it has become an either/or argument, and I am not too sure it works that way. If we are all capable of engaging in heinous acts, the exact variables would need to occur at the exact right time.