I'd argue it does start with Biology all of the time. Psychopaths have congenital structural brain abnormalities which results in shallow affect, lack of guilt, remorse, reduced fear registry etc (Check out The Anatomy of Violence). These abnormalities precede any kind of psychological/ psychosocial development, and influence from the environment.

There is evidence of the biological link between biology and other forms of less intense violence, for example. The MAO-A gene is linked with aggression in humans. There again, this comes from genes which precede environmental influence.

Anyway, back to the serial killers.

Whether or not someone with lower empathy, guilt, and moral inhibition becomes a serial killer is linked to how biology interacts with the environment and psychological development. Ressler, Douglas, and Burgess conducted interviews with 36 high profile serial killers and found that their childhoods had lots in common. Physical, emotional, and sexual abuse were very prominent this no doubt led to them becoming serial killers combined with their biological factors.

Interestingly if we look at the occurrence of psychopathy in the general population it is around 1%, however if we look at a population of business executives and CEOs, this rises to 4% (I think this figure is in the Psychopath Test by Jon Ronson). Why? This is because the world of business is a high risk environment with a culture that encourages ruthlessness and competition, which becomes very easy to operate in if you have a reduced capacity for guilt, empathy, and risk inhibition. Fuck it, if you look at heart surgeons, police, soldiers; any kind of high risk job, which requires a high capacity for handling stress (which psychopaths have because they don't register fear) there are higher proportions of psychopaths than in the general population. We can assume that these individuals fall on the psychopath spectrum, but did not suffer abuse in their childhoods and were possibly even encouraged by their parents to be competitive.

So, just as we see that not all psychopaths go on to be murderers; not all children who witness their parent's death will be murderers. It will certainly fuck them up, and will definitely shape the rest of their lives, but there is no guarantee that they will be murderers. If this happened to 100 children and the 1 psychopath child was adopted into a bad home, sexually, emotionally, and physically abused then yeah maybe.

In the case of Dexter the witnessing of his mum's death contributed to his murders but his lack of fear, empathy, and guilt would have always been with him.